What kind of difference in speed on the speedometer would stock 205/45/R17 at 855RPM vs 215/35/R19 at 832RPM have? Any way of fixing the difference, if noticible?

 

If your spedo was reading 65 mph you would be actually running 66.8 MPH.

The 205/45/17 has 857.38 revs per mile VS the 215/35/19 having 857.38 revs per mile. I wouldn't be too worried about the difference unless you drive on the edge of a rolling felony.

Try a tire calculator like the one at Discount Tire. It's lots of fun.
www.discounttire.com/dtcs/infoTireMath.dos

 

Rough Approximation:
205/45/R17:
Sidewall Height=(205*0.45)/25.4=3.63 in
Diameter=17+(2*3.63)=24.26 in
Circumference=3.1415*24.26=76.21 in

215/35/R19:
Sidewall Height=(215*0.35)/25.4=2.96 in
Diameter=19+(2*2.96)=24.92
Circumference=78.29 in

Assume 60 mph as a point of reference

60 Mi/Hr=1 mi/min=5280 ft/min=63360 in/min

205's
@60 mph, RPM of tire=63360/76.21=831.4 rpm

215's
@ 831.4 RPM, Speed=831.4*78.29=65090 in/min or 61.64 mph

So a 2.7% difference which we could have got directly from the circumferences ==> (78.29-76.21)/76.21*100=2.73%

On other vehicles I've owned (GMC Pickup's and SUV's), I've been able to adjust for tire size via a power programmer hooked that reprogrammed the computer through the OBDII port. Not sure how to do it on a Mini though. I would think that a Mini service department would have tha ability, I just don't know if they would. Couldn't hurt to ask.