Sure those guys can improve (BTW, OLinemen like Person, Jones, Rhaney, Bonds, etc... can improve with more seasoning, too).
IMO, no matter what the situation, no team with a pass rush like the Rams should have so many QBs with such high completion percentages. Now experience may help with the high completion percentages. But, experience and ability will do helluva lot more.
McGee was only the #3 CB due to a lack of numbers at the position. While he has good speed, he had shown nothing to date which probably played a role in why they drafted both Gaines and Joyner.
Joyner is an undersized player with questionable hands who AT BEST is limited to be effective in the slot and even that is debatable. He might just be much better off at safety.
Both Jenkins and Johnson are set to be free agents after this season (Again this year's draft isn't just about this year's season). To date has either shown to be guys this team must re-sign? Can either shadow a team's #1 WR well enough to make an impact on defensive calls/play?
Can the Rams win with the current group? Sure. But, an upgrade at the position could do wonders for Williams defense. As I recall, his best defenses have had at least 1-2 good press man cover guy(s).