While I commend your intent for improved safety, I take real exception the suggested method. A few problems:

Which mfg's recommendations do we have to adhere to? Are we obligated to adhere to a mfg's recommendations that doesn't even produce a safe product to start with? Would this mfg engine restriction exempt the mfg from any issues of airframe defects..."you had a bigger engine than we recommend even if it was dialed down (prove it) thus it is your fault".

The size of the engine is not what makes it unsafe. If I want to fly a dialed down P160 in my Flash because it's the only engine I have, am I forced to buy a new engine?

Is it safer to have a P120 that can't make it beyond 200 S&L even if I can get it well over that in a dive?

Safety is not a function of the size of the engine. It is the pilot. Anyone who is flying in excess of the 200 mph safety code isn't going to be influenced by a "restriction" that they must adhere to every mfg's recommendations on what engine they should put in it. And I think the best way to maintain safety is to stop buying cheep products with know safety issues and enforce the AMA regs that exist.