[QUOTE 4596344, member: 21629"]Well my knowledge are about 8 years old. But as I mentioned I had a bike with Campa 10sp groupset. Veloce shifters and rear derraileur. Swaped to wheelset with Shimano 105 cassette and even no need to adjust gears, worked perfectly.
I had rear wheel swaped minutes before road race when I noticed broken spoke. I was asked one question - "how many sp?" and was given spare rear wheel with 10sp Shimano cassette. Again - no time to adjust gears, started the race and no problems at all.
But as I said - that was in year 2008. Newer groupsets maybe work not so perfect.[/QUOTE]
Nothing much has changed in 8 years but it's worth saying that sometimes it works just "OK", sometimes it doesn't work well at all, some people are more tolerant of poor shifting than others and my definition of poor shifting may be different to other users.
I've had users tell me that their gears are perfect but I've had to downshift 2 and upshift one to get some of the sprockets and I have had instances where the shift got progressively worse across three or four cogs, missed one completely and then shifted (badly) across the rest - and was told that was OK - because the user had never had a bike set up the way it should be. So expectations do vary a lot.
One business i am lead mechanic for provides professional Service de Courses for amateur and professional races as well as for some well-known sportifs and Gran Fondos - and if push comes to shove in a race situation, sure, we'll often put a Shimano 10 into a Campag 10 bike, or vice versa, ditto 11 - but we also know from doing this, literally thousands of times over many years, that shifting is always compromised. Some riders notice it, others don't - it depends on their previous experience and as noted, their expectation (which is based on that previous experience). There is also blind luck involved, too - if a wheel is replaced with one where the top sprocket just happens to be within a small amount similarly positioned to the one that came out, the replacement could work almost as well, assuming the chain isn't too worn ... but riders can be unlucky too and that's why experienced racing cyclists are always cautious about selecting the biggest or the smallest sprockets after a wheel change in a race, unless the spare came from their own team car - the potential is always there for a problem to occur.
What you can guarantee, simply because the physical dimensions of the components and the way systems work don't allow it to be the case, is that you won't get the performance that the makers expect in a mixed and matched system, whether the rider notices that's the case or not.
... The S-Works is my summer commuting bike and the Foil will be my 'fast bike' .... 
