My opinion-keys to getting the best sound are:
1. Having an amplifier with at least the same RMS wattage as the speakers. Double RMS sounds the best, but you have to be disciplined with the amplifier gain to not pop your speakers.
2. Add a subwoofer and use the amplifier crossover to cut out the low frequencies to speakers (I always like 200-250 hz).
3. Dynamatting behind the speaker.
4. Soldering your wire connections.
I do not feel there is a big difference in sound between an aftermarket 5x7 and aftermarket 5.25. A 5x7 might make a little more bass and will be easier to install. So my advice: get 5x7 and if you are not happy then do the steps above.