On anything bigger than a park flyer, you want to time your flights anyway. You can't afford to just plop the plane in the grass wherever the LVC kicks in. Power to get back to the runway, and full power for a go-around is a necessity. Having a programmable LVC is not as important, as you should never be flying the plane to LVC anyway. You've already got a rough idea of how long you can fly on 2600mAh. If you put a 5000mAh LiPoly on the plane, it's perfectly valid to assume you can fly roughly twice as long, but it's a good idea to "test the waters" by flying a few minutes, landing, and checking voltage. When you start getting down near 3.0 Volts per cell, add up the total time, and you have your total flight time. It's a good idea to "sneak up" on that flight time over the next few flights, gradually extending your flights each time and measuring voltage immediately afterward.

Converting from NiMH to LiPoly involves all the precautions that Latch66 described above. Having a programmable LVC is not bad idea, just not absolutely necessary IMHO. Latch66 mentioned that each LiPoly cell is rated at 3.7 Volts, which mathematically would tell you that each LiPoly cell is equivalent to three NiCd or NiMH cells. In practice, we abuse our Nickel-based cells because they can take it. Under load, they typically show about 1.0 Volts per cell. LiPolys can't take it, and when properly sized, their voltage depression is much less severe. They will hold around 3.5 Volts per cell under load, and in practice, are approximately equivalent to 3-1/3 to 3-1/2 Nickel-based cells. Two LiPolys in series are equivalent to a 7-cell NiMH pack, and three LiPolys equivalent to 10 cells.

I would assume you're running the typical 10-cell E3D setup, since you didn't mention cell count that I noticed. That means you need 3-cell, or 3S (S for series) LiPoly packs, with enough of them connected in parallel to handle the current drawn by the motor at full throttle. How many in parallel will depend on the brand. I would guess given that the E3D has traditionally been a 40 Amp, 10 cell plane, that you're drawing somewhere in the mid to high 30's for current at full throttle.

While I can't recommend a specific brand, my calculations are based around the Kokam 1500 cell, which is a pretty good deal for an 8C cell right now. For your purposes, three 3-cell Kokam 1500 packs in parallel, or a 3S3P configuration would be perfect, giving you 36 Amps continuous (well over 40 Amps for short bursts) and 4500mAh capacity