1QuickCoupe, whoever may have infered to you that the orginal HOTPIPES GB was plug and play was way off mark. As detailed explicitly in the post, the first GB was only a y-pipe and uppipe, the CHARGE PIPES, which run TO the turbo. The uppipe runs long, and requires the end user to cut it to length and weld up the appropriate ******. The downpipe, the pipe that runs FROM the turbo, is what we are discussing now. It bolts to the turbo to route the exhaust gases back under the car into your catback (it runs under the TB and down the small space available over the rear of the transaxle).
shoclownas far as sizes are concerned, a 2.5" DOWNPIPE is more than enough to push some BIG power (the ypipe is 2.25" the uppipe is 2.5"). I have made 430whp with my 2.5" DP, and I feel it would well support even more. But if your HP goals are very high (over 475whp), I would reccomend the 3".
The 2.5" DP will fit without much trouble. A 3" DP will likely require the use of a section of oval piping, which will be the main reason for a price increase. Most (if not all) T3/4 turbos will not require a 3" DP. Even though my current 62-1 has a 3" connection for the DP, I necked it down to 2.5". I was initially concerned that the DP was a resriction at high rpms, but it was actually my original turbo (a T3/4 with a small .56 A/R). With the current turbo (62-1 with a .81 A/R), I make power all the way to redline, with very little drop off (within 20whp of max at 7300rpm). And I feel that the drop off is attributed to tuning more than DP size right now.
So, to review, the 2.5" DP will be plenty for almost all applications. Something else in your setup will fail or prove to be a restriction long before a 2.5" DP will. But, if you have a 'money no object' build planned, and have the turbo and engine capable of pushing upwards of 475+whp, then perhaps you will want a 3" DP.
