The MixPre has advantages in a couple of areas. The MixPre has greater gain than the FR-2LE preamp does. This can be useful in nature recording, maybe less important in catching audio for use in film. The MixPre has a very good headphone amp, better than the FR-2LE has. This can be a big help when monitoring the recording. And the volume control of the FR-2LE is annoyingly small. The case of the MixPre is strong metal. The FR-2LE is durable plastic, but it's still plastic.
Doug Oade makes a Basic Mod for the FR-2LE, $615, and a Super Mod, $730. Both make a good machine sound better. Another option to check out is the Marantz PMD661 with the Oade Super Mod, $669.
Flintstone