I'm not a TP or FTE, but I've been involved in experimental test flying of helicopters (MoD) and much more recently, gyroplanes (for CAA). I've directed two gyroplane campaigns in the last nine years and on both occasions hired a professional team of two TP's (one non-flying in support) and an FTE. The value added is immesurable, both in terms of safety and technical input.
I know that CAA have been considering for a number of years (and it hasn't come to fruition yet, if it ever will) a scheme for the UK gyroplane community where a select few would attend a seminar (only days long) covering test methodology and philosophy, report writing etc. These individuals would then be type-rated for experimental work (the VPM M16 guy couldn't do RAF 2000's for example).
I've got very direct experience of one gyroplane type that passed BCAR Section T's dynamic stability requirements when test flown by a CAA-approved pilot (not a TP, but a guy I have a very high regard for in the UK gyro community). When flown by a TP, the result was a fail (AND the TP's kneepad data was almost spot-on with the on-board recording system). I'm not saying that the TP will always get it right, but my experience is that our man always did. And we got beautiful post-flight reports from him as well.
I imagine the number of caveats/restrictions/limitations likely to be placed on non-professionals by the CAA is likely to undermine any campaign to the point where it becomes of questionable value.
I know that gyroplanes are a tiny fragment of the GA scene, but I think my experience is transferrable and I'm afraid, my experience is a negative one. If you're going to do experimental work, you need a TP/FTE.
Hope this is of some use to the debate.