The crux of the matter is that if you can not make - whatever you change - totally and irrefutable compatible with the existing compilers (see comments on Ultra51 above) all bets are off.

Of course, if your "improved chip" was compatible to the effect of "if you do not specify 'expanded' it will work standard" all tools would work, but as the "advanced features" were not used, you would have a cost with no benefit.

If the '51 world were to drift off into "x has these instructions, y does not, but have these" the result of incompatible devices would be that the tooling cost would go up dramatically or - even worse - we would have to accept the manufacturers tools.

One of the glorious facts about the '51 is that there is competition on the software front and that software is made independent of the manufacturers. If you have seen some of the $#!+ some manufacutres give away/sell as "tools" for their proprietary architecture you would understand my "purist" attitude.

Erik