This is where we don't have to guess as much anymore, thanks to the specialized websites of Seahawk fans. They've already done the roster mirroring and provided the guys who fit. It is exponentially more sophisticated than what round a guy was picked, and whether or not he was a 2-way player. I don't mean any offense by that. I'm not capable of doing the analysis these sites have done. As long as it's out there, and free, we might as well read and absorb.
The closest parallel to Sherman this year supposedly was a cornerback from Fresno State named Riley who went undrafted and was signed by the Titans. Apparently the Seahawks didn't love him because they didn't select him, although they did pick two defensive backs very late who fit their criteria, and supposedly they love them as prospects. The general manager Schneider is so enthused about one of them -- Tye Smith from Towson -- that he hinted he had a projected comparison to a current NFL star but didn't want to say it, to avoid putting too much pressure on the kid. Seattle websites already investigated the matter. They think the comparison is Joe Haden.
http://3sigmaathlete.com/2015/04/27/the-seahawky-cornerbacks-in-the-2015-nfl-draft/
Again, not all of this is pure metrics. Desire plays a huge role. The Seahawks want self motivated guys. That's where the Dolphins deviated this year with the choice of Jordan Phillips. He had the highest percentage of body fat of any player at the combine, nearly 31 percent. To quote a scout from Nolan Nawrocki's draft preview guide: "He didn't play with a consistent motor. That was the reason they didn't play him every down -- stamina and intelligence....If he played hard, he is a first rounder."
Lippett does have nice arm length for a cornerback, above 32 inches, unlike McCain at 30.5.
You can find Lippett's test numbers on the table at this link. He didn't perform like a special athlete. I looked at it quickly the other day. He lagged by one or two tenths in all the categories you can't afford to lag by one or two tenths.
http://3sigmaathlete.com/rankings/wr/