I own an Impact which I have modified to the extent of selling/exchanging the shrouded barrel, the air tube and regulator/gauge unit. These were replaced with a KevG regulator with 'MFR' nozzling, a standard Rapid barrel and Huggett moderator. I also fittied a pressure gauge in the underside of the block replacing the blanking plug. The only difference between it and the standard Mk 2 late model Rapid (with scalloped loading port)is the anti tamper which locks the hammer spring preload adjuster in place by means of a broken off bolt. This means that when I change calibre-something I do fairly often-I have to perform the necessary adjustment via the regulator.
If you were to blindfold me and offer me an Impact or a standard Rapid I don't think I could tell the difference. They handle identically.
The only real difference to me is the RAW action where the bolt is replaced by a side lever cocking action which is a real improvement although the RAW block looks huge.