Some (very specific) situations in recent memory that I have encountered where I really enjoy min spatk flamethrower over min spatk lava plume are: flamethrower ohkos standard skarmory, flamethrower always breaks zapdos's subs, flamethrower 2hkos rotom (lava plume can with max damage twice) , and specifically for this team since heatran is a good response to gengar (none of the other pokemon are really valid here...well maybe forry) you can get the ko 10% earlier with ft which is (1 lorbed attack faster) usually priceless considering how badly gengar rapes this team.
But overall the reason I would suggest the change from lp to ft is that I have never thought that "I really wish I could burn <pokemon> on the switch" where as there are certain situations where heatran has barely grabbed KOs.
But you could put it down to personal preference. I will admit that burning Tyranitar would be priceless for this team.