This isn't the exact same situation, but in a similar vein...I'll offer this real-world experience.
I once worked for a client that had 2 full t1's, and a 768k t1 in a multilink bundle. The fragmentation and reordering done by PPP was keeping the CPU load at %80+, and the jitter was terrible. Breaking the bundle and dedicating the 768k for videoconferencing and doing per-packet load sharing on the 2 x t1 for data fixed all the probelms they were having. (It also increased the data throughput for backups significantly.)
-Ejay