I had a 118D and also drove the 120D before I bought. The extra power was noticable but only when you planted your right foot, the rest of the time they drove vitually the same. But the gearing on the 120D was totally different to the 118D, more performance orientated. Thus even with the same power mthe 118D would ahve driven differently.

By remapping you will loose your engine and power train warranty, that is not really sensible on a car costing over £20000.