HI! I'm having trouble accepting that my GPU issues are by design, because they didn't exist until recently. Can you elaborate on some things?

That is a thermal throttle and you'd be better off throttling down than up if you can get it to just sit at that rate first.
As far as I can tell, I don't seem to be overheating at all. The highest I get during constant play is mid 70s celsius, which should be fairly benign for a rMBP. Are you telling me that the GPU is now throttling on purpose at fairly safe temperatures? Why is it only NOW doing this and ONLY with FFXIV? I do not have these problems with any other games.

You need to remember that mobile devices can NOT have their thermal throttle reduced otherwise there will be severe damage to the motherboard.
Until this problem began, I never touched the GPU, so these issues are not a result of me futzing with the settings. I'm only doing these things to test possible solutions. I also never attempted to raise my GPU's clock settings above what it can naturally do. I just really want it to stop cycling down every 2 seconds.

Also, in quad-core systems (especially in hyperthreaded mode) when all 4 cores are active, the top clock rate goes down. So for example my i4770 goes from 3.9 to 3.7ghz when all 4 physical cores are in operation. That is by design.
According to my task manager, my CPU load never goes above 40% while playing FFXIV. I also installed MSI Afterburner to track my CPU cores and they all seems to be doing well, performance and temperature wise. It seems to really just be the GPU rapidly cycling between my P1 (765mhz) and P5 (660mhz) power states.

If there have been any firmware updates, or bootcamp updates since the last time you played, there may be cooling parameter changes that weren't present the last time you played. That's all anyone can really offer any insight into.
I have not updated my firmware or bootcamp prior to these issues starting. I did update some bootcamp stuff in an attempt to fix the problem but nothing changed.

I am definitely willing to try whatever you can recommend! Can you explain to me how I am supposed to throttle down and how that will help with performance? Thanks!