Mike, you shouldn't notice any difference(mainly because there wouldn't be any)if you added another amplifier unless the maximum power capacity of your 7500 is being exceeded(unlikely). As was already pointed out, another amplifier would use the pre-out of the 7500 to drive it and the internal amplifier channels of the 7500 wouldn't even be connected to those speakers, so the power would be the responsibility of the new amplifier alone.
As to the use of all channel stereo, this is sometimes suggested for a more uniform sound all over a room in party situations, etc., but it isn't realistic to have the same sound coming from all different directions. If you like it, of course, it's your money and your call, but I use modes such as DPLII, Neo:6 or Logic 7 to put the other speakers into more realistic use with 2-channel material.
As to the power used in all channel stereo, you're correct that the surrounds get the same sound and use the same power as the mains on that side. However, you may be overlooking the fact that when two speakers play the same sound the result is 3dB louder than before. For example, if the front left speaker is playing at a fairly loud 85dB level and using about 1 watt, and you then add the left surround at the same 85dB level, yes 1 more watt is used for that speaker, but the sound level from the two combined would be 88dB. If you kept the same 85dB combined level, each speaker would use 1/2 watt to play at 82dB and produce the combined 85dB sound level, and total power usage would be the same 1 watt as when just the front speaker was used. So, don't be concerned about extra power usage in that mode.
