I did a cat/mufflerectomy on my Beetle. It's loud, but still quieter than say - a stock Cummins 12/24v(pre-commonrail) truck, a pickup with flowmaster mufflers, a "riced" out Honda, etc. It's a very nice tone and volume level if you like hearing your engine. In my experience, keeping the stock exhaust pipe diameter does not produce a harmonic drone at highway speeds either, but I can't speak for larger exhaust pipes.
Boost gauge - for mine I tapped out a port in the solid plastic intake pipe between the intercooler and intake manifold. It's easy enough to do since all you need is to release the hose clamps holding it in place to work on it outside the vehicle. Spray out any plastic shavings with a water hose and you're good to go.
As far as the turbo itself goes, you may have the dreaded sticky vanes that plague these VNT turbochargers. There are many detailed diagnostic techniques on this forum for checking vane operation - but - on the Beetle it is very hard to get in there and see the vane actuator move. What I do, is pull the vacuum line leading to the VNT actuator from the N75 valve and either apply direct vacuum from the pump (crude but effective) or use a vacuum tool (MityVac or equivalent) on the vacuum line. I also typically pull the charge pipe between the intercooler and intake and listen/feel the flow from the turbocharger as vacuum is applied. If you do not have a vacuum tool you can rent one from a retail auto parts store or pick one up on the cheap from Harbor Freight.
Here is the N75 valve (firewall, driver-side), and which line to remove:
[
NOTE: The white capped valve is the N75 for VNT operation, next to it will be a black valve (N18) for EGR operation, disregard this one. Remove the vacuum line
VERY CAREFULLY as the hose barbs tend to break easily.]
With the engine running and at an idle, apply vacuum. The engine should bog down a bit , and you should hear the turbocharger spool up. The bogging/spooling should be pretty linear to the amount of vacuum applied. If the spooling/bogging is delayed, does not return to normal when the vacuum is released or makes no changes-- then you have vane actuation issues. Vacuum should not drop off once applied, if it does - leaky vacuum line/connection or actuator.
Keep in mind, a vacuum tool can achieve higher vacuum levels than the engine vacuum pump - so if you do have sticky vanes this may free them up but "pop" them into a locked position so make sure you repeat the vacuum/release procedure until the actuation seems fluid and linear, in essence exercising / sweeping the vanes and releasing carbon buildup.
If the turbocharger vanes happen to pop closed (spooling) and stay that way once vacuum is released, you're in for some fun. Time to clean your turbocharger vanes. There are many schools of thought on this as far as what works, and what doesn't -- but typically removal of the turbocharger is required.