I agree that TMX is probably the closest you're likely to come to FX today. The tonality is not the same, but much closer than Acros. I suspect that the right developer might make it a pretty fair substitute. I've got another bulk roll of Panatomic X, so may do some direct comparisons just for fun. FX seems to have a somewhat unique look, but it would be interesting to see whether that really shows up compared to TMX. A developer that improved the acutance of TMX might help; it is sharp but doesn't always have that crisp of a look. More smooth looking to me than FX.
To me the speed difference between TMX and FX doesn't seem all that large really, maybe a stop or a touch more.