I'm not sure Barca would fare as well in the PL, or Madrid. Just my opinion, but La Liga is very attacking based, whereas PL is a mixture of attack and defence, perhaps more defence. The physicality of the League would not suit certain players of Barcelona or Madrid.
You have guys who were never considered among the very best in La Liga like Silva, Mata... (even Michu
) tearing it up, there's no reason to believe Messi, Iniesta and co. would have any problems adapting. It's not like defenses in La Liga don't try to stop Messi, they just can't do it, he's also been the CL's top scorer 4 years in a row so it's not like European defenses in general are faring any better.
How many slams will Monaco and Ferrer win combined?
The better question would be how many Slams can the rest of the field sneak in with those two juggernauts around? Not many I'd say. The gates of Vina del Mar, Acapulco, St Hertongenbosch and Buenos Aires - the real Slams - are pretty much unbreakable for the rest of the field at this point.