If you swing it, the STi 6-spd is a fantastic piece of equipment and includes the limited slip front diff and DCCD. HOWEVER, you will need the following equipment:
1.) STi transmission, shifter linkage and fork, etc.
2.) STi half-shafts and hubs
3.) 4EAT driveshaft
4.) R180 rear diff in the case of 2004-2005 and 2007+ STi donor IIRC. I think the 2006 may be able to retain the R160 rear diff... someone may correct me.
Exactly what is done to the car (you say "highly modded" but some may consider my car "highly modded" while I do not)? What numbers are you putting down? What numbers do you HOPE to put down? I am interested in torque here. You have a few options here:
1.) Another 2003-2005 Transmission -- Good for about 315wtq, but depends on driving habits here
2.) A 2008+ WRX Transmission -- Purportedly contains LGT gears, so it should be good for 350+wtq
3.) STi RA 5MT gears -- Good for about 325-330wtq, but it seems that the gearing is mostly responsible as the gears are not much physically different from the 2003-2007 WRX gears
4.) LGT 5MT gears (the LGT transmission will not bolt-up since the rear diff ratio is different) -- Good for 350-380wtq
5.) PPG (Pfitzer Performance Gearbox) gears -- Top notch, good for 500+wtq, but VERY pricey. At this point, you might be better off with an STi gearbox.