Will I feel a huge difference going down to 10'' wheels? Will my top speed change significantly?

The difference is in width chage, not diameter.
A narrower tyre will have less wind resistance than a fat one, which will make up for the smaller rolling diameter of the 10, so end result is any top speed difference will be negligable.

Acceleration and braking on the other hand will be improved.

most of the 13" 175x50 tyres are slightly taller overall than the 10s... but it`s usually only a pinch, so yes you`ll probably notice it a tad, but yeah just a tad

the big benefit will be the suspension response & handling (feeL)

& then there`s the best bit <what i call> the "Flingability"

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No offence intended here but--> anyone writing a book about minis 30 years ago may not have experienced such worn or stuffed-with components as we are finding these days.

You should put your heart & soul into everything you do.

God I hope they don't look bad inside my arches... it's now my only worry..

Mike
A set of 10x6 should fill those arches nicely

Put your tyre sizes into the calculator to get your rolling diameter:
http://www.ausmini.com/forums/viewtopic.php?t=12115

Your top speed will probably drop about 2mph

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Too many cars, and too little time.