if you're immolating multiple targets for aoe when you have FnB then you're doing it wrong as you dont get the 60ish% dmg reduction from CR or the dmg reduction you get from the live version of FnB and with the 15% dmg buff(can be increased to somethig like 30% through relics) to incinerate and cast time reduction from artifact traits, not to mention incinerate got like a 40ish% buff in legion(basically think mist lvl incinerate aoe but with greatly increase dmg), you shouldnt need to cast immolate at all for aoe and if you feel hellbound to cast immolate eventho you dont need to, you can still use wreak havoc for it.
as for mana tap, i completely agree, its a crappy talent but it does have its merit(on demand extra dmg for a relatively big cost) even if it dooes feel a bit expensive for such a small gain but the cost can luckily be worked around as its based off current mana but its still additional gcds(i assume) that needs to be used, if it was off gcd i would be very happy.