Your best bet for peace of mind under high torque/HP punishment would be to go to an
0A6 tranny aka MQ500.
It will likely cost the same at the end plus you'll gain a 6th gear and proper TDI-matched ratios with no funny spacings of ratios. Having recently driven a China-made van while overseas, I can attest that it is no fun when the ratios are so badly chosen that the vehicle could not negotiate a moderate grade (in this case in 3rd) and downshifting to 2nd would put the engine near redline. In the TDI case that's usually the top and second-top gears (in reference to what you want to do
here).
Wow, that 0A6 transmission looks like a sweet option if you had the money to spend on sourcing one and finding all the other parts necessary to make it work in our MKIV cars. Hopefully down the line someone will install one in a MKIV car.
6 speed I know it is beefier than 5 speed but it adds up quite a bit of weight to a tdi that is already front heavy, guessing you will add up between 100-120 Lbs, starter, axles, clutch, flywheel, heavier than stock, I know most like it but I do not, having the extra weight off set my goal, so it is out of the question.
getting back to cryo why is it then that in the racing world people cryo their trannies and other components in a car? waste of $?
Where are you getting your numbers from on the added weight of a 6 sped transmission compared to a 5 speed?
From what I've found it's only a ~17 lb difference going from 02J to 02M.
Weight of 02J 95lbs:
https://forums.fourtitude.com/showthread.php?1303547-Who-knows-the-weight-of-an-02J-(or-02A-for-a-rough-idea)
Weight of 02M 112lbs:
https://forums.vwvortex.com/showthread.php?4664520-I-Was-Able-to-Weigh-these-020-o2o-02A-o2a-02M-o2m-transmissions-Transmission-Weight
112-95=17, so it's not nearly as bad as you make it out to be. Heck you relocate your battery to the trunk or install a lightweight Li-Ion battery and you've solved your problem. That's why I have the battery relocated in both of my MKIVs.
Your plan to put .658 5th gear with no change in the other 4 gears will go over very poorly on the road.
TDIMeister makes a really good point on the gap you'll have going from 4th to 5th if you install a taller 5th gear. I thought I saw you were going to change the R&P to 3.16 too?
the rest of the kit that comes with the 6 speed makes it heavy plus the tranny itself too, 6 speed with LSD really expensive too
Again you really need to do some more research before you write off the 02M 6 speed transmission.
You can get a 5 speed to 6 speed swap from Frans for $1450.
http://forums.tdiclub.com/showthread.php?t=434593
Then depending on which LSD you wanna go with there's either a $75 difference between Peloquin 02J and 02M LSDs.
https://www.fourseasontuning.com/02j498005a-peloquin-02j-limited-slip-differential.html
https://www.fourseasontuning.com/02m498005a-peloquin-02m-differential.html
Or $200 difference between Wavetrac 02J and 02M LSDs.
https://www.fourseasontuning.com/10309180wk-wavetrac-differential-02j-mk4-golf-jetta-5-speed.html?mfg_id=1098
https://www.fourseasontuning.com/10309190wk-wavetrac-differential-02m-mk4-golf-jetta-18t-24v-vr6-w-6-speed-non-r32.html
So lets say you can sell your current 5 speed setup for $700, you'd have to pay an extra $825 to upgrade to a much stronger 02M with a Peloquin LSD in your transmission
Max,
I respectfully suggest you look at the Eurotune 6 speed conversation kit.
I did the first one in the U.S. back in 2012. I sold my B4V with the Eurotune 6 speed conversion. Zero problems in 6 years. See my signature for my front wheel Dyno resultes
I have a brand new, never used kit I will sell for $1000, if you are interested
I was going to mention this kit, so you could keep your 5 speed transmission and then still have the benefits of a 6 speed transmission. See if you can find someone in your area that has a 6 speed in their TDI and see if you can drive it to see why so many people upgrade to a 6 speed transmission.
Now if you're still really stuck on keeping your 02J transmission, here are some upgrades I've found, but be prepared to open up your wallet and spend $$$$.
If you wanna have your cake and eat it too, you could always go with the 6 speed Dog Box Kit for the 02J for 3450 Euros which is approximately $3,988.12 at today's exchange rate.
http://www.sqsracing.com/produkt/176:197:vw-gearbox-dogbox-racing-gear-transmission
Then there's the 5 speed syncro kit for 1570 Euros or approximately $1,814.88
http://www.sqsracing.com/produkt/276:362:synchro-box-racing-competition
Then you can't forget the sequential shifter for those upgraded transmission
all for the low, low price of 1470 Euros or approximately $1,699.16
To make sure you know what gear you're in, you better get the Gear Indicator for 50 Euros or $57.79
So if you went with the 6 speed Dog Box Kit, Sequential Shifter and Gear Indicator you're looking at 4970 Euros or $5,744.62 to have the ULTIMATE 02J transmission in the world. The 02M doesn't look to bad now does it.
Here are a few other options I found as well.
http://store.forcefedengineering.com/searchresults.asp?cat=134
http://www.sqsracing.com/stranky/27:gearboxes-complete-dog-vw-audi-01e-02a-02j-02m-02q
https://www.ecstuning.com/b-cts-parts/02a-02j-pinion-brace/ctshw054~cts/
http://holingeramerica.com/showroom/mf-2wd
Then this company does metal surface treatment on transmission parts, LSDs, etc, so you'd have to contact them on what you want done and if they can treat that kind of part.
http://www.wpctreatment.com/about.htm
Good luck on your decision, but my vote is to upgrade to a 02M with a LSD and never look back.