Have just flown my first 210 with gear doors in the past few weeks, and would have to say it is definitely slower than those that have had the mod done. Probably in the order of 5kts IAS. Could be just the machine though...
The extension/retraction cycles are obviously longer, which is more annoying than anything, waiting for that little green light. If anything, having the gear doors makes the aeroplane look a little smarter when it's on the ground (and probably in the air too).
I hadn't flown a 210 for quite a while up until a fortnight ago - brought back some sweet sweet memories
TL