You're most likely correct. Don't know why Horace started focusing on turbines, as I will wager that the original poster didn't have turbines in mind when he/she asked the question since a turbine pilot is quite likely to have been briefed on turbine rules as he/she got the requisite waiver.

To get the original poster's question back on what I perceive is the correct track, how about rephrasing it this way...

Is there an AMA speed limit for fixed-wing non-turbine RC or control line planes?