The added unsprung weight and inertia of the larger wheels can have negative effects on handling, braking, and acceleration. Acceleration will not be as good as with the 16's due to the extra weight and intertia of the larger wheels....dyno tests have shown actual horsepower losses when changing to larger wheels. Handling will also be effected slightly because of the added unsprung weight and the fact that you'll have to run such a low-profile tire on that size rim will make the ride a little more harsh. You'll also have to watch for curbs and pot-holes with big rims like that.
On the other hand, just bumping up one size, to say 17's, shouldn't effect things nearly as much...especially if they're light-weight rims. I switched from factory 14" steel rims to 15" aluminum rims and I was able to reduce understeer noticeably (due to the stiffer side-wall), but there is a little more road feel transmitted through the steering wheel...and not exactly in a good way.
Running a rim of 21" on a vehicle that originally came with 16" rims, can also cause premature ball-joint, tie rod, and strut/strut mount wear and/or damage.
New Toy- Stone stock '88 T-bird Sport in need of some TLC
1988 Cougar XR-7- HO/T-5 conversion, cracked block prevented from reaching full potential.
RIP '88 T-Bird....firstname.lastname@example.org...best sleeper in So. Jersey, now it's taking a permanent dirt nap.