I have the Mishimoto as well and although I have not measured what the can has caught I would say my results are similar to @GV27 or possibly a little more.
All right fam I have like 1,600 miles on my FL5, its been sitting for a bit due to the winter weather here in WI. I checked it and it has caught this much amount of gunk. Definitely very small, but also not "nothing". You can actually buy this catch can on Amazon for $20 (it's called Bull Boost, I think Mishimoto is an exact copy of theirs with a huge markup), and it even includes the mounting bracket. You just have to buy a small section of fuel hose as well. You can even reuse one of the oem hoses, see my pictures.
I added some scotch brite steel wool to the top section of the can, a $1 mod. Hopefully gives even more surface area for gunk to attach to and disperse back to the bottom eventually.