There was an interview with one of the SPARC creators who said that a huge benefit of control of the instruction set was the ability to take the platform in directions that (Intel) resellers could not.

He was otherwise largely agnostic on the benefits of SPARC.

ISA shouldn’t be more than a 10% or 20% performance impact or so, per jim Keller

What this means is that a better ISA might permanently be one generation ahead in terms of performance

That's not trivial...