That's right. There is a list of advantages of BlackBerryOS in this thread: http://www.blackberryforums.com/viewtopic.php?t=1995
One important thing to keep in mind about those slow 7750's, is you must make sure you're not already too low in memory before installing BlackBerryOS. Make sure you remove unneeded applications and delete old emails ("Delete Prior"). Since the hourglass can suddenly show up excessively on a 7750 when it runs out of memory (which happens much more often on 7750's than other models of BlackBerries). As long as you keep an eye on the memory remaining, you should be OK with 4.0 on a 7750.