Van Jefferson easily could become another Kupp. Like Kupp, Van is a technician who separates because of his crisp routes with a burst coming out of his breaks. His long speed is in the 4.6 range meaning most DBs are faster. But his route running and burst gets him open.
Wes Welker is an excellent example of player development. He went undrafted because he was undersized and posted only average speed. Originally signed by the Chargers he was released after one game. He spent 2004 through 2006 with the Dolphins. He was about to be cut when he was abruptly traded to the Pats and the rest is history. It was with the Pats that he got the level of coaching he needed and blossomed. He also continued in the offseason at a private training center in the midwest. There he improved his running style and went from a 4.65 to about a 4.55 which may not seem like much but that coupled with improved route running and understanding defenses is where he excelled.
Drew Brees got benched as a starter for San Diego. He deserved getting benched his play was poor. On the advice of Doug Flutie, he began to train offseason at a center in Carlsbad, CA. He told me that one of his exercises was to look at a room for 5 seconds, then have someone move one or more things. Drew would again look at the room for 3 seconds and then again back out of the room. He would then say what was moved and where it was before and after. It was one way he trained his brain and eyes to read the defenders pre-snap and to be aware of how the defense might change post-snap.
Kris Dielman a ProBowl LG started as a failed 3T on defense. They switched him during TC. His first year he was horrendous. But each year he improved and his play. By the end of his 3rd year, he was considered a rising star blocking in front of LaDanian Tomlinson. So absolutely players can make dramatic improvements, but IMO it's a matter of the player's commitment and work ethic. Kris was the player that told me playing the o-line or any NFL position was all about technique. He said less athletic players could play at a high level in the NFL if their technique is good.
I still say WR is a lower priority. This is a golden opportunity to use Everett out of the slot with Higbee in his usual spot. Two TEs in the short area on each side of the field occupying the LBs, with Kupp in the mid-range and Woods behind him going deep. Now add in Henderson in the flats, play-action, or just a power run between the tackles with the LBs out of position because of Everett and Higbee. If the center takes out the MIKE Henderson can potentially take it the distance. The problem is if the center is Blythe who struggles to pull and block on the second level then Henderson gets 5-8 yds. Go ahead and break down Blythe's second level play. IMO he misses more than he makes. If Allen can't go I hope Shelton earns the start.
Bottom line is that any WR they take will likely get limited snaps barring injury to a starter.