[6ren]

Yes, I think a limit on productivity amounts to a limit on knowledge. Does knowledge have a limit? There's no evidence for it; all our present mathematics of complexity suggests not (but it's possible one will be discovered...)

However, energy needs do seem to increase with productivity, and there's limit on how much energy we can get from the sun (a dyson sphere encloses the sun, capturing all its energy). But there's some assumptions here: that we can't discover ways of being productive that require less energy (and wouldn't that enhancement itself be "productive"?); that we can't discover other sources of energy (e.g. room temperature fusion micro-suns - or something better).

Once you factor in the unknown (i.e. knowledge that we don't yet have) it's hard to know what the limits are. I guess the entire universe is a limit on matter... if we can't discover a way to make more matter... or more universes.

[Symmetry]

There are actually mathematical limits to the amount of information that can be stored within a given volume, the Bekenstein bound, which is a lot like a limit to knowledge.

[6ren]

Well... if we can make new universes, the volume isn't limited.