normal box is good for about 250bhp.
The sadev unit is fine, its not only full throttle change, and down changed using the cluth makes life very easy. Only downsite is transmission knock and judder, the whine and in traffic use.
You wont get neat the 11 sec 0-100 time.....your missing slicks and have far too much weight. Gears alone dont mean accelleration........my 4 spd XR2 is faster to 60 than the 5 speed MK2.
IF you can prove a need for the cupracer then theya re cheaper form the facotry than a std 172, we were selling 172s for about HK$150,000, cup racers were about $110K.