Voadam
Legend
At level 20, where this feat gives its maximum benefit, you only get 20 extra HP. Improved Toughness is best on casters or anyone else with tiny, D4 hit dice, and even then, casters have better option (Faerie Mysteries Initiate allows you to use Int instead of Con Mod for calculating HP for wizards, for example.)
Do I need to point out the flaw in the above?

the same post said:Take for example the CR 20 Tarresque.
. . .
Incidentally, I believe the Tarresque chose Toughness for all of its feats?
If the 48 HD CR 20 Tarrasque were to use non PH feats and used Improved Toughness instead of toughness his six toughness feats would instead of being +18 hp leading to 858 hp would be +48 hp each leading to 288 hp an increase of 270 for a total of 1,128 hp.
For a different example take a CR 8 18 HD Tyrannosaurus. Advance it to 51 HD with one size upgrade. That is now CR 20 and has 11 new feats to spend. And if you want to use a couple of them on improved toughness and swap out the three toughness feats for improved toughness it gets to be very, very tough.