First off Reynolds is at a minimum a decent backup and has been productive when given the opportunity. If Van Jefferson doesn't beat him out in a limited training camp that is not necessarily a big deal.
It is fair to question the lack of productivity of Van Jefferson. He did play against the top college competition and played in a poor offenses so there is also a reasonable expectation that in the right offense his production will improve. There seems to be a consensus that Van has decent size, great hands and great route running skills. It is a fact that he has had years of pro coaching from his dad. There seems to be some question about his speed as he did not run a 40 at the combine. However his on field speed at the Senior Bowl was timed as the fastest player there above Mims who ran a 4.38 40. So it seems as he is plenty fast enough.
There is a lot to like about Jefferson but he did go late in the 2nd round, recently had a foot injury and lacked the college production that many of the receivers taken ahead of him and behind him had.
Comparisons to Kupp: I think the comparisons to Kupp are fair in the area of being pro ready coming out of college. Kupp was like a coach on the field and had great knowledge of running routes and getting open. Jefferson has had years of pro coaching and has great knowledge of running routes and getting open. From a route running perspective they are both very advanced coming into their rookie years. Both also have great hands. Kupp ran I believe a 4.62 40 and it seems like Jefferson is probably faster than Kupp. Where they differ is that Kupp college production is off the charts while Jefferson production is really not very good. The level of competition for Jefferson though is significantly higher than that of Kupp.
The big thing is that both are pro ready which means the floor for each is very high. I doubt many predicted Kupp to have the ceiling he has achieved when he came out.
How good will Jefferson be? What is his ceiling? We know he is pro ready. We know he can run a pro route tree and has great hands. We know he is fast enough and may be the Rams fastest WR. We know he is going to a diverse high powered offense with a good QB. I think there are good odds on him being very productive though he may be brought along slowly because there are 3 good WR ahead of him and training camp will be abbreviated.
I am excited about the pick and at a minimum I expect big things in year 2.