Its cheap and simple to fit. Take about 30-40 mins to do if you have an haynes manual...even quicker if you are a proper tech
Loui is right it sounds like its a boost issue. If it was ECU then you'd have lights on the dash or the faultcode reader would pick something up. Oh and if the MINI goes into economy/limp home mode it does about 40mph in 5th or 6th after taking about 2 miles af hard driving through the gears to get there. I will not go over 2k rpm! ... so it can't be that and the EML/Check engine would be on