I tried a frost build with TTW, water elemental and ignite on the PTR and it is weaker than frost/TTW. I suspect there was a build on the WOTLK Beta at some point that Blizzard thought did extremely strong DPS with FFB, so they toned down some talents and made frost-based frostfirebolt builds not work all that well.

Not having to put 5 points into improved frostbolt frees up a lot of flavor points early in the frost tree.

I think the 3-tree hybrid is an interesting concept and if mage talent trees are revised in the future, it's a build to keep an eye on. I got a warning (near-ban, I suppose) on EJ because I said that it's an interesting build even though it doesn't do very good DPS.