I will concede your point about Lendl's FH though, since I completely forgot how useless Sampras' FH was on clay. I only thought about the running FH and general offensive capabilities he had on that wing.
I think the idea that Lendl and Sampras had FH's that were "similar on fast courts" grotesquely undersells Sampras's. Pete's had nothing like the consistency of Lendl's, but it wasn't designed to. It wasn't designed as a rally stroke, per se. But nonetheless, was designed and implemented in such a way that it was almost as critical to his fast court success as his serve was. The difference was just in the way he used it.
Pete's was a FH designed to go for winners on fast courts. And he used it to bludgeon opponents. While his serve and net game made him perilously hard to crack when he served, his go-for-broke Howitzer FH made it almost impossible to hold every game in a set against him on courts that wouldn't let a big eastern FH sit up after the bounce.
It's not a strategy that gets used any more (since fast courts are extinct), or appreciated by newer viewers, since the idea of inconsistency as a weapon seems patently absurd in world of Djokovics and Murrays. But Pete's results on fast courts (7 Wimbledons in the fast court era) and medium courts (5 U.S. Opens) speak for themselves.
The US in particular (when it was still a little faster than it is today) was an excellent showcase for all kinds of strategies. Clay courters like Wilander, power baseliners like Lendl, and flat out attackers like Pete could all gain traction there. But on true fast courts, Lendl and his array of weapons never even broke through once. Doesn't mean he was inept there -- he was very good at Wimbledon. But he certainly wasn't great. And we're comparing him to the greatest fast court player of all time, and a guy whose FH was a devastating weapon on that surface.
I'd say Pete's FH was as much bigger a weapon on fast courts than Lendl's, as Lend's was over Pete's on clay. On medium, they had pretty even results.
Pete, of course, had a little more success overall than Lendl did on each of their preferred surfaces. But then, Lendl's forehand was a much bigger percentage of his overall arsenal.
I'd call it a push, all in all.
No doubt at all that Lendl's would look better on today's tour, however.