People throw around "diminishing returns" badly sometimes. If you start @ 100 of a figure, add 10, that's a 10% increase. Add another 10 to 110 and if you use 110 as the base #, you'll get a figure of less than 10% increase. But it's not, any increase should be compared to the original base #.

So there is such a thing as linear returns and as far as prime stats, det/crit/acc, give linear returns. Until cap. I've heard #s thrown around for a crit cap that was 520 ish I think? Haven't heard of a det cap, and acc varies by job and is easily googled.

skill/spell speed are a different ball game, and are effectively increasing returns with a cliff at the end.