the "conservative" (as you say) or one-shot amount for one 220 film is, according to Fujihunt instructions, 313 ml for 100 ASA, or 417 ml for 400-1600 ASA film. (Calculated from the table linked above.) That's still far from 1000 ml.

In any case, professional labs usually use replenishing procedure. The replenishment ratios are much lower than that. AND, replenishing is also a process that needs control. I would bet that many labs, even some calling themselves professional, just replenish without very strict control, maybe running a test strip once in a while, and that's same than just extending the dev time. But when extending the time, you do it only few times so it cannot go far away.

The point is that these one-shot tank volume minimum values have enough "safety guarantee" to develop the film to strict densitometric specifications regardless of exposure (eg.: underexposed roll exhausts the developer less rapidly and leads to overdeveloping with low solution volume), film type (some oxidize developer and/or release bromide/iodide more rapidly) etc. In a replenished system, the large volume of tank solution keeps this under control, and on average, the given replenishment ratio (per film) keeps the process somehow under control. So, to emulate this effect in one-shot environment, a bigger volume of developer can be used.

HOWEVER. If you know that your roll is properly exposed, and you know, that the results you get from this film are satisfactory to you (you can even do a test strip with it and measure!), there is no reason why you would need so much developer.

And, extending the time is a cheap but still completely usable substitute for replenishment at home. It's not 100% same as replenishing, but very comparable. But then, of course, there's the REAL limit for the chemistry. Look at the replenishment ratios KODAK gives! They are quite close to how many rolls can be made by extending time in small-tank processing.

Reusing by following the Fujihunt instructions WILL NOT cause any severe problems you would be feared of: it has no effect on grain, film speed, shadow detail, or anything like that. The chemicals are meant for these volumes and will work, but the exact densitometric control may be a bit off. But remember, it may be off to begin with because the differences in agitation style and tempering.

If you expose properly or take that in the account, I would guess you cannot see any difference in one-shot or two-shot or even three-shot, using the volumes given by Fujihunt.