FAA requires hi-performance endorsment for 200+ engines.
Is that requirement only for certified planes?
One CFI indicated that it is not required for experimentals.

Depends on the operating limitations of the aircraft in question. There
is an FAR somewhere that says, in effect, you don't need the appropriate
ratings to be legal -- legal that is, not safe. However, frequently the
operating limitations for that individual aircraft state differently.

Ed Wischmeyer