If by 'high performance' you mean 'on par with A-Bombs' well, I don't know how technically feasible that is - certainly if you get into 100+ kt - but I would imagine that they may well have a lesser stigma attached to them, certainly in terms of fallout. This could lead to more nuclear power being used worldwide - possibly meaning more disasters, possibly not. I imagine that thermobaric warheads may well be used for peaceful purposes, as nuclear devices like India's Smiling Buddha were ostensibly for. These peaceful purposes would probably be mining and canal construction - in the latter case, a canal to the Qattara depression using nuclear weapons was on the cards at one stage, so that might be possible!
In terms of weapon development, I imagine this may push the earlier development of MIRVs, since in order to flatten a city you'll need lots of smaller bombs compared to one single large nuke. (Okay, theoretically you should have lots of small nukes anyway, since they spread the destruction more efficiently.) Of course, thermobaric warheads of similar scale to smaller nuclear devices would still be enough to wipe out entire towns and small cities, so they're still pretty lethal on their own.
So putting all this together and applying it to history, I would suppose that MacArthur's request for nukes would be substituted for somewhat-less controversial thermobaric warheads, and might possibly be allowed. Of course, this could then mean many more near-WWIII situations, and if the world actually got on the bad side of one of these then the destruction would be horrific. The one ray of hope would there would be no nuclear fallout from the bombs - but of course, that could always come from the more numerous power stations ATL...
If you can isolate a POD for this, then I imagine a very interested timeline could be constructed on this premise...