On 2.5" dp/tp and stock IC/inlet you are going to notice a significant increase in lag. 3" dp/tp and 2.5" piping and the increase in lag becomes negligible.
I've done quite a bit of back to back riding/driving in my 400whp NA geared stock turbo car and a 500whp TT geared 2530 car. If my stock turbos ever go I will definitely be on the look out for some 2530s or something similar. You really need the 3" exhaust and 2.5" piping though. Before breathers the lag was really noticeable in the 2530 car. Lag was minimal on the stock turbo car before breathers and after it's pretty much non existent, especially with the gearing.