Sounds like you've never really tried to onsight at your limit Jim. Its not for everybody the megapump you can get on a limit onsight but if you're into it, surely this is part of the attraction, barely sketching up some horrendous sequence by the skin of your teeth by a made up sequence with your eyes bulging out of your head? If everything was nice and comfortable and within your limit and you didn't get pumped then the outcome would be a lot more certain but you wouldn't be at your limit I suppose. In some ways, I can see how it would be more attractive just to settle for a quick RP. Far more efficient to bolt to bolt it, rest 30 mins then smoothly send, JStar style. Can't be much of a buzz tho surely!
I remember in the 90's JB Tribout discussing in the mags (in the OTE Moon vs the World article in 95 if memory serves) the ability to onsight 8a (or flash 8a+) correlating with 8c and onsighting 2 full number grades below a person's RP limit has been widely corroborated in many countries as a kind of world standard for a theoretical 'average' climber (holding true for the lower grades too so RP 6c means OS 6a and RP 7c means OS 7a). The calculation is for a climbers with plenty of onsight practice, trying to onsight routes at their limit.
Taking the UK, as has been discussed earlier, the white/grey rock and lack of practice of many UK redpointers at onsighting means that many (myself included) do not fit this scale in this country. Surely we don't need a specialist UK scale to calculate our theoretical onsight, we just need to recognise that we haven't practised it as much as the Euro's and/or don't have the routes available to practice on or simply don't like onsighting and prefer RP'ing (as did Wolfgang Gullich). The option is there if we choose to get more practice in on our Euro travels. BTW, not all UK routes are desparate to onsight, Defcon 3 is just as onsightable as any 8a in Chulila IMHO. All the holds are well chalked and the line quite obvious.