My experience of solar lights is that they're not immensely bright. I've seen many of them, and I have several of the garden ones myself, and while they have their uses, none have been bright enough for porch lights. They're better as markers for the edge of paths and drives. Also, the amount of sunlight hitting them each day affects their output quite a lot, so unless your porch is very bright during the day they're going to be even less functional. I'd hate to sound like I'm saying solar lights are bad, I personally like them a lot for the right uses, but I don't feel they're going to be good in this situation.

Besides, since your current porch lights are such good quality and you're happy with them it doesn't sound like you really need to replace them at the moment. You also have mains electric connected to them allready, so going with solar won't save you the cost of wiring in new lights. However, if you're concerned with the amount of electric they use then there are several low cost things you can do to improve them. Replacing the bulbs with low energy bulbs, such as compact flurescent bulbs (be sure they're suitable for outside use as some of them have trouble starting in the very cold weather) will cut down their energy usage a lot. 11w flurescent is approximately equal to a regular 60w bulb, so the potential savings are large. Also, installing a mechanical timer so they only come on when needed can help. More complex alternatives are motion detectors, or light sensors that automatically turn the lights on when it's dark.