He may not be able to actually "get faster", but his 40 yard dash and shuttle times absolutely can improve markedly over a year with training. Plus, the whole idea that LJ is maxed out is asinine. Look at LJ next to CJ. CJ looks maxed out, LJ isn't ripped like he'll be after another year focusing with Yancy.
NFL teams have scouting departments, but the NFL is light years behind college offenses in how to deploy unusual players. It's not a good idea to rely on the NFL to notice how an unusual multi-tool player like LJ can be deployed. Some teams will see it, some won't. That's the way the league works and it will cause his draft stock to be lower than if everyone saw it.
Most WR in his spot should probably go pro, but LJ has a unique skill set that distinguishes him from all other WR candidates. He'll get ample chance to demonstrate that if he comes back and it will cause his draft stock to be much higher next year. LJ is going to be a monster next year in a monster offense with coaches that know his strengths and how to deploy him. The media puff pieces, especially on his versatility are going to help his exposure to those same uncreative NFL teams immeasurably.