Hmm...
Hard to say from my experience. I would say that change to 4hp wouldn't really hurt. It is 3 flux building with 5hp, that is a lot and it's base in the same time. What's really the difference between this and Magma Chamber? Salvage (well, useful in certain conditions) and +E domain. For 4 flux of difference, and Magma Chamber cannot give you Mogi cards.
So, yes, I would say that building could be changed a bit, though I don't see that much of problem coming from the fact that for 2 flux you can get a card. Every Mogi costs 3 flux and is not that superb (probably except Cinder but if you have one deployed, all others will be just plain fighters with average stats), for 5 flux they are expensive for what they can do (if you would like to use Sleet Mogi with his ability you would pay 6 flux for him and 3 flux for next ones; this is 1/1 with aloft, I would still choose Thistle Falcon).
So the spamming problem is not that big and will not be big once Elementals will not be able to have 2 flux wells in 3 rounds
But his price for base, ability and 5hp is not really right.
Btw: does anyone playing mono Elementals use Magma Chamber?