I speak as a person who knows next to nothing about the regulations which apply to the modification and racing of warbirds. I also know very little about P-51's. However:
surely flutter testing would have to be part of ANY test regime following these modifications. Surely this testing needs to be completed prior to ANY racing in front of grandstands.
Gosh, I hope so, But I wonder...
These intrepid aviators invest fortunes in these aircraft to get every knot of speed out of them. That obviously involves modifying them. Were it to be a certified aircraft, there would be very specific criteria to which the modified aircraft would have to demonstrate compliance - by test flight.
I'm guessing that's not the same in respect of these racing planes (but I don't know for certain). I'm imagining a fellow with his pride and joy super fast modified warbird, going to the races to wring every knot out of it he can. I'm not imagining that he takes the plane to the races and says: "Yep, here are the flight test results demonstrating a flutter margin up to 550 knots, so I'm going to fly the course at exactly 500 knots, so the margin of safety is there for everyone. I'm not passing judgement either way, I'm just guessing that it's just different from certified flying most of us know.
Most of us get in our beloved and equally maligned "spam can", see a red line on the airspeed indicator, and think to ourselves: "Gawd, I can't imagine having the nerve to even take it that fast!". Be assured that model has safely flown faster. Every aircraft I flight test, I fly to 110% of the red line speed, as required by the design requirements. This is of course done in accordance with a flight test plan, under the terms of an experimental flight permit. But it is done so that when someone goofs in the cloud, rolls it over and pulls through, they are safe, if they don't fly it past the red line (and don't over stress it). Try to imagine how far the nose has to be put down to get a Citabria float plane past red line! It's not going to happen by accident! On the other hand, you can get a Caravan or Navajo through fairly easily....
The military back in the day subjected the P-51 to intensive flight testing, and I'm sure there's lot's of data out there. But from that, I'm betting that they established appropriate limitations, and red lines. When you start clipping wings, adding power, and flying it extra fast, it's no longer a P-51 out of the box, it's a new plane, with a lot of P-51 parts in it. Things can be different.
None of this is intended to portray the racers in a poor light. I'm confident that they apply the resources needed to produce a plane which is appropriately safe. I would if I had that much invested! They know they have the design standards to fall back on, if they need guidance for the modifications.
I'm just having trouble imagining the same margins (110% for speeds, or factors of safety of 1.5 to 2) between "proven during testing" and "flown in service" that we are used to with certified aircraft.
I wish the racers safe and happy flying (and little regulatory interference), I am in awe of what they do. More than in a lot of corners of aviation, I trust that they do things carefully, and well. But, I also think they are probably flight testing around those pylons too...