The Accord and TSX exhaust manifolds are not the same length. A while back, when the Accord and TSX were new, there were issues with aftermarket headers cracking because they had a one size fits all approach to both cars. I know it's not a huge difference in length between the two, but it was enough to cause damage over time.
Unless you're planning on replacing your entire exhaust or modifying it, you're better off getting an Accord header. Actually, if you're just going to swap the header and keep your stock exhaust, save your money and spend it on something else. An aftermarket header won't help much with the stock exhaust as a bottleneck.
Good luck with your plans.