might sound nice at first, bt is it really usefull or even needed ?
with bonus 5, they can transport 6 stacks, eg 1200 wires, circuits or science (not even looking at increased stack sizes from other mods like 500 or 1000 for bricks, concrete or walls). when using 100 drones (or maybe 144 or whatever that limit is), this results in the depot storing up to 120k or more or much more, which probably wouldn't work with the current implementation. and by adding infinite research for capacity, you would also need infinite storage in requester depots ... besides requester depots needing an increased capacity, sooner or later also provider depots wouldn't be large enough to satisfy requests at once and probably deliveries would be deferred for a long time until the requester amount drops to partial stacks when the contents is less than 1 stack which would cause quite erratic behavior. thus there has to be ANY limit anyway.
in addition to this problem, probably nobody would want this option, and shouldn't be able to accidentally research high values since ALL requester depots would ask for insane amounts of resources: although 20 stacks of 50 ore might sound nice (to get 1k ore per drone, up to 100k ore for a depot with 100 drones), this at the same time would cause the smallest requester depot with only a single drone for science to try and fill the depot up to at least 20x200=4k and up to 8k science (or similarly insane amounts of nuclear fuel, coal for a single burner, radars, powerpoles, etc) ...
thus unlimited capacity probably would be a bad idea, at least until there is also an option to limit the number of stacks on a requester (similar to inserters' option "limit stack size to xx"). i think that many people would like such a soft limit already even for the current max bonus of 5 :-)
in contrast, a huge speed bonus (at least if it results in speeds slower than one tile per tick) should at most cause problems for UPS, but not for any hardcoded limits of depots, pathfinding, collision detection, etc.
