I see two small flaws with arguments made on both sides of this debate:
1) Linking the quality of bowling faced during an innings/career to the career records of said bowlers - bowlers have good days and bad days. Bowlers rely heavily on the pitch provided to them. It's very possible to have scored a 100 against an attack of ATGs that was easier than one scored against a lesser attack, due to the ATGs having an off-rhythm day bowling on an unhelpful pitch, and the mediocre bowlers bowling at their peak in friendly conditions. This has to be taken into account when you talk about the quality of innings played by a player - an example that comes to mind is Binny's 80 odd in Trent Bridge. Down the line in statistical arguments it will look like an fifty scored in the 4th innings of a test in England against an attack of Anderson/Broad/Plunkett/Stokes and Ali. Not bad...except when you consider how dead the pitch was and how jaded Broad/Anderson were; bowling nowhere near to the quality their records suggest they can.
Similarly the quality of bowling the English batsmen faced from the Sri Lankan bowlers was significantly better than the career records of the bowlers would suggest.
2) Assuming overseas conditions = alien conditions - You have seaming pitches in India, Bangladesh and Sri Lanka, you have dry turners in England, and you have roads pretty much everywhere. Vijay's 100 in Trent Bridge will appear as a 100 scored in his first Test in England and down the line can be taken as a sign of his versatility and ability to score on foreign conditions out of home...but to all of us who saw that match we know the pitch was more Indian than anything in India even.
These are just recent examples off the top of my head; I'm sure if you go back and do enough research you'll find many more. So instead of talking of how many tons were scored overseas or against 'ATG' bowlers, maybe focus on how many really good innings had been played - in pressure situations, against good bowling (regardless of how the bowlers' eventually careers turned out to be), in tough conditions (regardless of the country the match was played in) I think you will then have a better idea of how good a batsman really is.