I spoke to Clive and a few others about this last year. Apparently the exhausts come to within about 10 thou of the bore! (or something ridiculously tight like that). So you can in theory get away with it with A1 valve guides, but I really wouldn't fancy trying it.
Once you get up to the +0.040 region, it's apparently a reasonably sensible modification. I think it was Alan Ramsbotton I spoke to about that.
I'm interested in going down the +0.040 route (which equates to about 2050cc). I don't see the sense in the "might as well" argument of going up to 2200 simply because Rover built an engine of that capacity. The history files suggest that the 2200 capacity is the leftovers from an aborted DOHC slant-4, which would have been half of a 4.4 V8 and fitted to P10. So it wasn't necessarily a considered engineering decision that led Rover to 2.2-litres for P6 - more a case of make-do-and-mend, and a marketing attempt to stay competitive against the European influx of 2-litre exec saloons in the mid Seventies. Any developments to P6 in these later years of its life would by their nature have been financially prudent quick-fixes, and far from the million-mile relentless test, tweak and retest exercises that were undertaken in 1960/61.
As such, given I would be sourcing NOS or new pistons anyway, I'd want to pick the dimensions I wanted rather than what parts commonality dictates is easiest to source. There are two reasons why I wouldn't want to get into the 2200 region:
The obvious is that I could retain the 2000 piston design and combustion chamber shape. I think a deep-dished piston with the squish area in the centre immediately below the spark is infinitely preferable to a flat-topped piston stopping 6mm shy of the top of the bore. Yes, there is the argument that they are 'bulky', but I don't think that will detract significantly in terms of inertia hammerblow on the journals or additional friction.
The second is that an increase in capacity necessitates an increase in valve sizes to maintain the same flow rate at a given rpm, so you're not necessarily gaining anything at 2.2 litres. If anything you're upsetting the balance. On 2000 head, the inlets are substantially larger than the exhausts. On the 2200, the inlets are unchanged but the exhausts have been increased a bit (though still far from the inlet size). So even if the increase in exhaust size is sufficient to empty the additional capacity on the exhaust stroke at an acceptable rate, there will be greater relative constriction on the inlet owing to its unchanged size.
I really feel that a 2200 TC head on an overbored 2000 block with Heron pistons is the only viable combination in gaining significant value added. Otherwise you're just hovering around single percentage gains over a standard 2200, which is perfectly acceptable but not necessarily a big enough return for the amount of work involved.
Michael