you can buy any tire you want that's true, but 225mm is about as wide as you can squeeze on to the 7" wide rim. And as I said before do that, and you will be running a smaller diameter, which my main concern would be the added mileage to the odometer. (I think it was a couple hundred miles per oil change.)
And yes not all 215-50-17's are exactly the same size (TireRack does list the true OD as well), I'm sure ASTM allows for some mfg. tolerance of around 1-5%, but I'm sure it will be a smaller delta then 225-45 vs 215-50.
The ideal solution to running a wider tire on the TSX is the 235/45/17, but it does require a new rim 7.5" wide. An 18" rim would allow for a TSX ideal 235/40/18 as well.
The upside is the much larger selection of maxi-performance models available in the 235 width. 215-50-17 is more passenger car oriented then performance car in it's tire choices.
And if the TSX is your daily ride, the new rims are not that large of an expense, expecially if you want maximum performance rubber. As those types of tires are not cheap $150-200.00/ea. Add in the life cycle of 10,000-20,000mi and the cost of new rims won't matter come a typical 4 years, 75,000mi of use.
My 225-45-17 Potenza's cost more (per tire) then the my BBS rims did (per wheel.) So in my case my rim was an accesory for the tire.
After all if you are purchasing maximum performance tires, you might as well shod them on some lighter performance alloys.
I'm finding the OEM TSX Pilots have a very long life span, even longer on ours since I run a seperate 16" alloy (215/55/16) snow package in the winter. That said when I do need new tires for the TSX a 7.5" wide 17" or 18" performance rim isn't out of the question.