Not only for speed, also for resolving power they are quite different.
With same technology, x4 the speed is x4 the crystal surface, so x2 the "diameter" of the crystal.
I'd not try to rate lp/mm of lenses using TMY because we would not see which LF lens is better, while TMX says it.
Another thing is if top image quality is important or not for an image...
IMHO it is true that in 8x10 film resolving power is not that important, but it's equally true that IQ of a good 4x5 TMX shot is comparable to IQ of a 8x10 TMY shot, nearly matching I guess.
Datasheet says extintion at 50Lp/mm vs 65Lp/mm at 1.6:1 low contrast, but they don't say the exposure, IMHO in practice difference is way larger. TMX is a very, very sharp film and TMY it is way less sharp.
Not that extrange, it can be suspected from the x4 speed difference.
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