I have yet to get back to audaxing and my 650b experience is from since my op (and I was only 200 standard before that) so with that rider on my observations
1 Using a 700c frame (that will take up to 28 tyres under the guards) the change in diameter has little noticeable effect on handling. The wider tyres do have a much bigger effect on bike behaviour. On the Peugeot that I converted the change to the handling made very little difference to the riding experience (28x622 v 32x584). On my current mtb conversion the 32x584 are quicker than 26x1.5 (easier rolling) but the handling difference is unnoticeable.
2 On smooth surfaces the 23x622 are much quicker. Once the surface gets more granular (top dressing broken up by frost etc) 28's are better than 23's but 32x584 are in a different league. 26x1.5 is not in my experience that much better than 32x584. For me 32mm seems to be a good compromise for comfort - my 650b bike is the first choice for route testing for this reason alone.
3 If you want to use 650B with rim brakes (swopping between 622 and 584 and using deep brakes) it is possible but the choice of rims is limited and can be pricy (this is especially the case in France
) and heavy (ditto). I have sourced 32h rims from SJS in UK because there is absolutely nothing available for the weight and price in France. If you are using much wider rims and tyres and disc brakes the choice is much easier. (Anecdotally my current conversion is using discs but the rims are 19mm wide and suited to rim brakes; I stopped using the Peugeot because I needed the rims for the tandem!).
4 There are (as in all things) good tyres and less good tyres. Even if there is an advantage in going wide a wide Compass tyre is still going to be streets ahead of a supermarket IRC demi-ballon (which is about the same size). My 32mm Hutchinson tyres are better than the IRC (and probably more comfortable). I would like to have the money to try the Compass tyres in a big section but life ain't like that. If you want to swap between big section 650b and 700c then it really needs a frame suited to the job and almost certainly disc brakes (if only for the availability of rims). There are gravel frames that would fit the bill. Whether they would be suited to the rider's use is another matter, of course.
My tyres are 32x584 Hutchinson Confrérie de 650B (which are light, quick, comfortable and relatively cheap, especially for members of the Confrérie; slick treads which I personally would not want in mud) and Schwalbe HS165 reinforced (no longer made I think) which are like a Michelin World Tour that rolls much better than the Michelin and lasts much longer. The Hutchinsons are kevlar beaded, the Schwalbe rigid
The next step for me will be to fit one of my old racing frames with deep brakes and 650b Hutchinsons and get back to a bit of long distance - 2020 perhaps?