Heat isn'tthe only thing that limits a card's speed - there could be other bottlenecks which are affecting hashrate.
I have changed vram and other card settings with the miner running to no detriment however be aware that the impact on your hashrate may not become apparent for a few cycles. What I've read online insists that with an undervolt and an underclock you're not at risk of damaging components, but you
are overclocking your VRAM so just because the slider goes that high doesn't mean it should be pushed that high.
Yes I paid above the odds for my 5700xt and with my 580 mining alongside it it is going to take some time to make that investment back over the retail price of the card, but remember if you break it you'll have no income whatsoever.
I don't think your monitor will impact your mining too much - mine is a 2k 144hz gaming thing and my hashrate is barely impacted. In fact, I'm mining right now with all cards with no input lag using lolminer, although I've seen some lag using teamredminer which, although it reported a higher hashrate on my screen, showed a lower hashrate poolside after a 24h run.
If your card has dual BIOS you can always switch to the BIOS you don't use much and try modifying that if your heart is set on it. if you bork it you can boot up in the working BIOS then switch the switch over to the other one and repair the broken BIOS (by re-flashing the original BIOS you've saved before starting) and then try again. I plan on doing it once I've figured out my VRAM thermal pad issue (see
other post) since mine has a silent and OC mode switcher.