from what I remember (don't have anything in front of me, but have worked on many of both harnesses), the EG33 and EJ TCU plugs are the same. so swapping between those TCUs would be easy. if not, getting my hands on the plugs will be quite easy, so 3 sets of plugs would also be doable
You're probably right, the connectors do look the same. I know for sure however that the circuit board of the Legacy TCU vs the SVX are very different. It looks like the microprocessor is a different variant as well, although they have the same instruction set.
I also want to dig through some FSMs, and compare gear ratios, otherwise when the TCU compares the speed to what gear it should be in, things will work out. (my buddy had that problem when he put a jdm wrx 4EAT in his impreza LX. it worked fine, except the gear ratios were all off, and then when he put the jdm TCU in there, it was expecting a 7k redline, but the ECU would only allow 6200.)
I have found the gear ratio map:
seg006:D74D GearRatioMap: fdb $B24 ; DATA XREF: CalcVSS1+4o seg006:D74D ; 1st gear seg006:D74F fdb $62E ; 2nd gear seg006:D751 fdb $400 ; 3rd gear seg006:D753 fdb $2C6 ; 4th gear
So it shouldn't be too hard to fix that problem, too. The TCU uses the gear ratio map to calculate the input shaft speed after the torque converter. This in turn is used for TC lockup and Duty C calcs.
Redline shift points are adjustable as well:
seg006:CDFF engineRedlineMap:fcb $5F ; DATA XREF: LookupShift+4o seg006:CDFF ; Gear 1 seg006:CE00 fcb $61 ; Gear 2 seg006:CE01 fcb $62 ; Gear 3 seg006:CE02 fcb $FF ; Gear 4 seg006:CE03 fcb $61 ; Gear 1, manual mode seg006:CE04 fcb $62 ; Gear 2, manual mode seg006:CE05 fcb $64 ; Gear 3, manual mode seg006:CE06 fcb $FF ; Gear 4, manual mode
For example, 1st gear $5F = 95 ($5F in decimal) * 64 = 6080 RPM
Another issue is with the differences in VSS signals; on the Legacy TCU the VSS2 was no problem as both the Legacy and the GL10 have 4000 pulse per mile. However it expects a 4.11 rear diff with larger tires so that ratio was off. I found the constant for VSS1 and corrected it. There is also a constant for the AWD slip detection that might need to be changed. Mine was only 6% off so it was no problem and I didn't change it.