and to

You need to read a lot in this forum to answer all 3 questions.

An XT with high mileage would be one with over 70'000 miles i.e. 43'505Km

A powercommander can be used on a restricted (by facotry) bike and will make very little difference and no, its not illegal but if the bike has to be restricted then you're better off disconnecting it until you are at the legal age to use it.

some models have a difficult to thame ECU and tend to surge a lot, however we have quite a few fix for those, again read the forum starting from my signature line on mods.

Good luck