Steve, have you had problems with your turbo wastegate leaking? Mine has just been found to be not sealing correctly I think this is why I have been down on power as boost doesnt rise too quickly.
You wont be down on power as peak boost will be the same (unless your wastegate is stuck wide open)
I would always go aftermarket on the ECU front. It gives you the flexability to make changes without incurring massive mapping costs. If a re-map by Fast Chip costs you £400.00 a pop it gets expensive as you'll need to take it in for a re-map every time you make a modification. Standalone gives you the option to modify / tweak your own maps for free (as long as you know what your doing).
Are you sure you want to go down the Omex route? I know it seems to be the ECU of choice amongst the Clio Tuners but I've found the resolution on the map to be quite poor compared to the DTA that I'm using now. Have you considered an Adaptronic, they get great reviews and also have a self mapping feature built in, you fit a knock sensor and set your target AFR's for different loads and it apparently does the rest for you. We are fitting one to a 225 lump in an R5 at the moment an its not up and running yet so can't say how user friendly it is but it sounds good in theory! A lot of people use them on the RTOC!
Suprised this project isn't finished yet, seems to have been around for ages now