IMO, strings make a big difference. Once you find a frame you can play with, the strings and tension either make or break the frame. It's been my experience that I play a ton better with Luxilon TIMO than any other string. I agree that is the player, but if you give a player two rackets identical rackets, one strung with a bad string choice and one strung like he likes it, the player will choose the racket strung like he likes it. There have been many cases of folks demoing frames that were strung improperly or with a string/tension they didn't like and as a result they didn't like the racket. It probably wasn't the frame, it was the string/tension.
I know that I have found the ideal set up for me. The C10 and TIMO 17 at 54 pounds (although when the V-Engine MidPlus comes out I will have to give it a run) is the ideal set up for me. Likewise, when I played with the Vilas, natural gut at 58 pounds was the perfect setup for it. I tried a couple of multis and natural gut was by far the better choice in the Vilas.
I know that I play better with that string/tension than any other setup I have tried. So, in my experience, it seems that having the proper string/tension is what makes all the difference in the world. The hardest part of changing frames is getting the string/tension combination right. The number of frames that you would ever consider buying is far less than the number of string choices/tensions that you can play with. Boris Becker once said that his racket merely held his strings. This implies that the strings make or break a racket. IMO, this is the gospel. A good string choice will lift your game through improved consistency and confidence. A bad string choice will send you into the land of demoing.