may not fit the description of a tuner, but the caddy cein greatly outdid my expectations (and my tuned 700something horsepower ford GT and my 700something horsepower Camaro LM race car). great braking, great acceleration, good cornering grip, and with tuning can hit above the 290mph mark (with NO2 of course). as for real tuners, I'd say the Suzuki GSX R/4, built to the hilt. only around 270 or so hp, but it can take on esprits and other MR cars and win like they weren't even there. doesn't look half bad either. the 85 Pajero Evo can also get up and boogie very nicely, and depending on how you have the VCD tuned can do some nitrous drifts (get a slide going then mash the nitrous button. works very well on the Escudo). RSC Rally Raid can hang with comperably powered sports cars on track as well as hold it's own on the dirt/snow/wet tarmac
yes, i know none of those are production cars, but they all exceeded my expectations greatly