I would put the fx6 on a shelf of memorabilia somewhere. The use of canisters went out in the 1980s (at least in my neck of the woods.) Sumps were coming into "vogue" and have become the standard. One I cannot argue with.
As far as heaters go, dropping them in the sump, or putting them "inline" are no different. There is no efficiency to be gained. It will take the same number of "watts" to heat the same volume of water.
What you don't have dropping them in the sump, is the plumbing issue. If you don't have a sump, it is time to start planning one. You can run a reef on a canister system, many of us old timers have. It is just a bigger chore, than it oughta be.