I bought the 128i because I thought it was enough power,(remember the 95-99 M3 was 240 hp and people though that was great), I really thought i needed a new car, and the economy sucks, so the 128i seemed to be way more modest than getting a 135i.
I was out the door for barely $30k with sport package and heated seats, w/ 0.9% financing and only $500 down.
Other real options to the 128i were of coure the base WRX, the volvo 30 hot hatch, the mini cooper S or works even,
I found out later the accord coupe has 263 hp, but it will always be a front driver.
So Now, I am sometimes wishing for more power;
But not enough to void my warrenty with an aftermaket supercharger, actually not even enough to pop $2k more for the performance intake and exhaust.
I'll probably get the short shiter, as I have done that with 7 of the previous 9 BMW's i have owned over my lifetime.
However to go fast, I went out and bought an aprilia Tuono 1000cc 130hp, 430 pound italian motorcycle. ( w/ a 200 lb rider thats translates to over 400hp per ton)
It was $5k used, and 0-60 is about 3.2 for the skilled racer and about 4 for mere mortals like myself.
So if i loose my job, I can always sell my motorcycle , even just for $4k, and have money to survive. If i bought a supercharger for the 128i, (or other performance parts) I'd never see that money again, and with the SC void the entire car warranty.
the 128i is a very nice car, and is better than the e36 M3 in some regards, and costs less NEW than i paid for my '95 e36 M3 certified USED in 99!!!!.
With the sport pack, I really think all it needs is the short shift and wider front tire to dial out the understeer, and/or probably the thick rear sway bar for the same reason.
After that you're just playing foolishly with your money.
enjoy the car for what it is, and put some money in the bank.