You could say that MF is capped, or, better yet, the more MF you have the less effective it is for uniques/sets/rares due to the deminishing returns. The actual "cap" on better chance at finding uniques is 350%. Having 350% MF means you only have 225% UF in reality. You need 10000000000000000000% MF to have 350% (the max) better chance to find Unique Items. The caps for sets/rares are as followed: 600% 700%
Now as for your question, I would ask you to look at the numbers: 2000% MF = 320% UF and 1000% = 295% UF. You be the judge. (Remember having 100% MF means you have 2X the chance of having that quality of item drop. So if you have 100% UF and your chance of a specific drop being unique .02 without MF, with MF it would be .04.)