And if TX can run x2 460 in SLI, would the HX model boost performance? Or just be a waste of money for these particular GPUs?
No increase in performance.
For $20 extra the 650HX gives you 80Plus Bronze efficiency vs just 80Plus for the TX. It also gives you hybrid modular cables. You can attach only the cables you need, giving you a neater interior, which is important to some people with windowed cases. And you also get the 4 PCI-e video card power leads.
There are things you can do to tidy up the interior of a case using the 650TX and the 2 extra molex-to-PCI-e adapters you'd need to use. You could wrap them in tape to more closely match the other cables, use shrink wrap tubing, etc. You can get those items at case mod shops like Xoxide or FrozenCPU