I personally don't think anything will be added to the block header within 5 years, since that would have the effect of making lots of existing ASICs useless. Also, if anyone wants to "add" some extra data to a block, they can already do this (in a way that's compatible with all existing miners) by using the coinbase.

What I mean by less-than-optimal is that since the SHA-256 constants are not hardcoded, some logic can't be optimised out at compile time. Within an ASIC, this would manifest as increased die area or power consumption. But I have no experience with ASIC design, so for all my ignorance it could be an insignificant 0.0001% increase or an embarassing 10% increase.