I am pretty-sure that at the high levels of skill development (past 20 ranks) you only get a +1 per rank*. So no +5 per level. This is the same as for RMSS/FRP.
Which means you can continue to increase the bonus but not discustingly so. I do believe that the returns should diminish, but not completely go away. One thing I really like about RM is the unlimited levels (in theory). Games that say "this is the best you can ever hope for" come across to me as artificailly limited.
* Isn't it: Ranks 1-10 (+5/Rank), Ranks 11-20 (+2/Rank), Ranks 21+ (+1/Rank)?