Bit late to the party on this one, but as I had two MPI's (one with, one without sportpack) I thought I'd comment.
My first one was a 97 MPI standard Cooper on 12" wheels. I had it a year and then was lured by a deal to swap it for a the 98 Sport LE (Brooklands Green with the wide tramlines). I loved the LE and though I think the characteristics of the handling were worse with the Sportpack, I did like the more planted feel and they looked great at the time. They drove like completely different cars. Issues I found were that it could be a bit skittish in the wet when driving enthusiastically - as Cooperman says, probably a case of not enough weight over each 'wide' tyre. By comparison I could drive the 12" wheeled car much more aggressively in the wet with almost as much confidence as in the dry. The performance of the LE with the bigger wheels also suffered, especially with the long legged final drive. So although I loved the LE, I must say the standard cooper drove better.
If you're going to have a Sportpack, I think they need the usual performance upgrades (head work mainly, and a road friendly cam) to get the torque and power up a bit within the limitations of the ECU, and also fit a different final drive. Some of the later 'S' type cars with claimed bhp circa 85 were much nicer to drive, and had enough torque to move those heavy wheels and tyres! I never did drive one with any more power than that.
I think the Sportpack looked good in its day, and I wouldn't mess about changing a nice original one back to 10" or 12" wheels, but if I was starting from scratch (as I am) then it would be 10's or 12's with narrow section all the way.
So now over 20 years later I have another MPI but will be going archless with 10" wheels, plus the same sort of performance mods mentioned above.