I've decided that I want to have Empower, Maximize, and Empower Healing all three on my Evoker FvS. I'm human so that gives me a total of 8 feats, 5 of which I didn't just lock in.
After those 5, I would say Toughness is unquestionably off the table for removal. In addition, Evocation Focus is off the table since we only have to remove one feat (which would be the Greater one). Finally, while Heighten has a smaller affect on Implosion it's the better choice for every other spell we have so it would get the nod in my mind at least over Greater Evocation Focus.
So that leaves our last two - Greater Evocation Focus or Extend. Extend makes blade barrier kiting, something which I use quite often, much more sp efficient on tougher fights. It also makes raiding much smoother - especially if I'm the one throwing down prayers and recitations - and allows me to perform the upkeep on divine might/power when I'm doing portals much less often. Greater Evocation on the other hand varies greatly on how important it is based on what I'm fighting. It could have 0 effect sometimes. It could mean a very slight reduction in how effective a spell is. Or it could mean my spell is twice as effective (5% landing to 10% landing). On the other hand, if my spell is 5% effective I'm not likely to be using it so I'd say reasonably the best I'd be seeing is a 10% increase in effectiveness from 50% to 55%.
Examining everything I'd lean toward dropping Greater Evocation Focus. It shaves a tiny bit of effectiveness off the top but trades that for a lot more flexibility which opens up more options for smart play to conserve sp's as well as reduce the amount of time I have to be recasting spells.