There's a trick a lot of the newer or mediocre investors do -- put tiny amounts into popular late-stage financings of known winners, just so the investor can claim "we were an investor in all these great companies."
Picking a great company at seed stage is harder. If you could have a seed-only portfolio where in 5 years someone knows even 50% of the companies (positively; not talking about investing in adelphia, etc.), you're doing amazingly well.
Based on yclist, I'd say YC, one of the better seed-stage investors, is probably around 10-15% by that metric, assuming none of the YC companies not on yc list are "known".