I think it is sensible to be cautious about mixing and matching components. When talking about these components it is difficult to speak other than broadly or generally. The specifics do make a difference.
Broadly speaking, the unique design of the Vincent offers the opportunity to modify the rake by changing the the length of the backbone and suspension. As mentioned, this has led to much better handling of many racers, but the specifics do matter.
Also, broadly speaking, the new steering stem has no effect on the rake of the bike, so there is unlikely any problem with doing the Reg Bolton mod. The great benefit of trying the Bolton mod is that it is inexpensive to do and can be easily removed.
I am not sure that there is a good reason to do any of these mods (except for the new stem mod) on a streetbike. I do understand Greg's interest in putting these mods on a streetbike, because once you personally become aware that you can make a Vincent handle this well, you will find yourself less interested in riding stock Vincents.
Race bikes have always been about experimentation and beating the bikes in front of you.
David