I'm still on 8gb of 1600mhz.
Thinking 16gb of 2400mhz and a 4790k would tide me over another year or 2?
Personally, yes. you have a 1080ti which stomps all over my 980ti and I run everything max at 1080/1440p at 60fps (Forza 7 @ 4K) no issues and the CPU has never been topped out and at most seen 10-12GB max RAM usage.
Obviously the newer cores will be more efficient and better at well everything, but £200 over £500-600 outlay, you are going to be getting brilliant bang per buck

Reviews like the one this thread is based show that the performance gap really isn't that huge.
I reckon I am 98% sure I will still be on this rig next year, might have an extra 980ti or a 1080 in it, but the same core