Here are the tire differences.
295/35/18:
Sidewall: 4.1 in
Radius: 13.1 in
Dia.: 26.1 in
Circumf.: 82.1 in
Revs/min: 772.0
255/50/16:
Sidewall: 5.0 in
Radius: 13.0 in
Dia.: 26.0 in
Circumf.: 81.8 in
Revs/min: 775.0
The speed with the 255/50/16 is .3% too fast. At 60 mph, that tire size would actually be traveling 59.8 mph.
http://www.miata.net/garage/tirecalc.html
Also, I do not think the later C4s had 16" wheels. I may be wrong, but I think the last of the 16" wheels were on the '88 models. The offset may not be correct.