Heighten shines when using lower level spells as It makes their Dc's harder to resist. Divine casters are generally going to use it for DC pumping as opposed to increasing the damage output. Spell focus feats are good for a few spells and add +1 dc to those spells and +0 to all of your other spells whereas heighten adds much more the lower the level of the spell. For examble, heighten adds a +4 to the dc to resist greater command where as enchantment focus would only add +1. +7 to hold monster dc, +3 to blade barrier etc. Also, you would have to take another feat in necromancy focus to increase the dc of your destruction by 1 whereas heighten would increase it by 2. In summary, if you care about spell dc's enough to take evocation focus etc. then you should first take heighten and then see if you are struggling to land spells. IMO its harder to break spell resistance in vale/amrath than land the actual spell.