The BRM rod journal is 50.6mm. The BHW is 53.7mm. But the center-to-center is the same going back to the AHU and is the same currently. Crank stroke is the same, although there are 4 differing cranks in the series. American version engines began using cracked rods in '04 instead of the conventionally made rods for the AHU and ALH. The real reason for this is the 'bean-counters'; cracked rods are 30% cheaper to make. With the cracked rods, also came the tangless, sputtered bearings. The rest of the rod dimensions remained the same, except for the 2.0l BHW engine, first in the series with the larger 'big end' rod.
As for offsetting crank: there is no point making a stroker out of this engine. The piston is virtually 'no clearance' at TDC to the head. Increasing the compression for performance? You are actually going the wrong way. The compression reduction should be at least 1 1/2 pts and more likely, over 3.
I think going to 82.5 on the pistons is not going to happen. The two Siamese pistons are going to be too close for comfort. The advantage for all VW diesel 2.0 engines, is the larger rod journal. If you want to match up a BHW rod to a crank in an ALH block, you have to get a BHW crank.
We do think this is a reasonable choice for a heavily modded engine, as smaller large-end rod journal of the ALH can be a problem. The other advantage of the 81mm BHW piston is the CR (compression ratio) is 18.25:1 instead of the 19.5:1 for the ALH. The Crafter pistons decrease the CR to 16.8:1. Before you become unhinged about that large a CR drop, all the common rail engines run stock centerbore combustion bowls that are the same 16.8. There are no complaints about cold starts as far as I know.
The valves for the BHW and BRM are interchangable. As a matter of fact, except for a very minor change in the cam, the cylinder heads are identical.