you took extra life away from your skis because you had it flattened out. they will ski fine with out the bases being ground flat... (at least this is what i have understood for previous discissions about this topic). By flattening the bases you took away extra edge that did not need to be taken away. In reality you would have to grind your bases MANY (as in 20 - 30+) before the bases were shot. I asked a tech about this a few seasons ago and he said that the amount of material that is taken off your skis is so minute that it makes little or no difference. Obviously that after several grindings you will notice a difference. By gringing your bases flat right at the start not only did you use up your first tune but you took away extra material from the edges, thus decreasing the life of the ski.
My take on this is that it will not make a significant difference in the life of the ski. If it were me i would have skied them 3 or 4 times then had the grind done - factory tunes are not always the greatest. I may have wanted to ski on the concave base ski first just to see what the difference was, but i would assume that it is minute, and only noticable if you are really engaging your edges on ice. As for your skis i dont think that you ruined them. By grinding too much material away of course you would ruin them, but i dont think that is what you had done. A normal base grind will flatten your skis out and is just a regular tune.
One thing that i have heard is that the grinders being that they are designed to take away edge as well as base material can take away the base material faster than they grind the edges. This would result in a concave base, so if it is an issue oboe, im sure that it would be adjusted and dealt with, although not my reccommendation. I wouldnt worry about it, when we tune our skis to race we take a file to them and make sure the base is flat - then we perform the tune.
Much of that is assumption, although i dont believe it to be far off base, i woudlnt lose sleep over it until you ski them and know for sure.
ps. going to bed now and getting up to go skiing