Based on test data I have seen in the past, even in all channel stereo the difference will be in the neighborhood of a couple dBs. Also, remember when you listen to 7 channel stereo you have all 7 speakers making sound so that will offset some of the burden on the amps. Your Klipsch speakers are all rated 96 to 98 dB/1m/W and nominal impedance of 8 ohms. Even if you factor in the possibility that Klipsch exaggerrated their specs, a mid range AVR should have no trouble driving them in a small to medium size room.
If you listen to music and movie so loud that you have to worry about not bringing out their full potential you may risk damaging your hearing. I would say if you are really worry about that, then keep either the XPA-5 or the UPA-2 but you don't need both. Or if you must keep both then consider lower cost AVRs such as the SR-6010 or AVR-X4100W, both cann process 9 channels and have Audyssey Platinum. They should be good enough to hold you over for two years, especially the 6010. Even with those lower cost AVRs you really still don't need the UPA-2 in a 7.2 HT setup, the AVR can drive the rear surrounds easily.