I don't want to call
@EdZ a liar, however, his numbers are misleading, since they do not account for overclocking (which can add ~50% power consumption), and are on the conservative side, therefore making his base assumption invalid.
Also, this power supply (800 LTI) is not of the highest quality and you wouldn't want to run it on the limit if you could avoid it (not to mention the abomination of noise it makes when pushed).
Some people on this forum think that 800W is a "holy grail" for sff even though it really isn't that much. I've been reading many of these power requirement suggestions and discussions and feel inspired to support the oposite of the conservative estimations regularely proposed.
I fully support your idea of adding an assist power supply as an experiment, however, this way you add potential sources of failure to your system, which is something you might want to consider.
Since you mention future bottlenecks I will quote some higher power components but the articles have lists so you can educate yourself. Whatever someone may say, maxing out the component power consumption is not that hard in real world applications.
7900X is a reasonable processor to be used even with the mITX board and it can pull easily over 220W, 250W+ peak. Even the 7960X and 7980XE can be used but then the overclock limitation would be the board VRM which wouldn't like over 400W (but I guess you would melt the poor single 8 pin connector and the cable before that) and 7980XE can pull
over 600W peak.
Moving on to GPU's. The non reference 1080Ti's can pull about 350W each when overclocked and 350 x 2 was about 700 back in school. Now, consider that a few months from now AMD releases a high power card you might want to upgrade to, the current Vega cards can pull a fabulous
450W when overclocked (with modded bios etc.).
The answer to your concerns is therefore dependent on how much you like pushing your hardware. If you enjoy the higher end of the overclocking spectrum, 800W is almost absurdly low.
The resources on which I base the previous paragraphs:
CPU power consumption
GPU power consumption