To be honest... for a great many people, releasing an SDK for SH3 now would likely cause a great deal of problems.
Modders have fixed a whole PILE of stuff... and using an SDK could potentially... and quietly, reintroduce many problems.
Had it come earlier... it might have been great. If it came today... it would likely result in "mod-soup" and all sorts of unforseen madness.
Its definitely one of those "be careful what you wish for" thingies.
