Hi there Mart,
As suggested by Trevor, I would stick with a crop sensor camera, especially as you're interested in nature / wildlife...
If you move to a full-frame camera such as the 5D you would notice the following:
1 - The crop sensor gives your long lenses a bit more length; the same lens used on the 5D would not be as long. So if you're interested in getting frame filling nature/wildlife photo's this would be easier on a crop sensor than a full frame sensor.
2 - A full frame sensor would give a shallower depth of field, a constant challenge with macro photography as I'm sure you're aware. A crop sensor would suit macro photography better.
3 - If you moved to a 5D you would need to invest in a new walkaround lens. The 18-55 is not compatible with the EF fitting on the 5D. I'm not sure if the 18-270 is compatible but my gut says no.
It might be worth looking into the 7D. This is a crop sensor and it compatible with your existing lenses. Also, the AF is move sophisticated than the 5D. However, before making any moves, why are you moving away from your 550D? Chris asked this already and is a key question. It might we worth investing in a flash for macro work or better lenses instead of changing bodies.
Cheers
Ben