MS45.1 can be tuned for forced induction, several companies have done it, ESS, AA, TT, etc. But, they don't know that DME as well as the MS43, and so there are some challenges when tuning it for higher horsepower. It is rumored that ESS didn't release their TS3 kit for the MS45.1 cars because of some issue they ran into with the DME at those higher power levels, for example.

Think of it this way:
MS45.1 - moderate understanding is available, enough to get by, but nothing fancy
MS43 - its an open book, everyone knows how to do just about anything with it