Thermal design power of the T7300 is 35W, while the X7900 does a 44W. So yes: it may become warmer (or the cooling fans wil work harder). On the other way: your SSD draws less power than a conventional harddisk. My 7.1 is upgraded since 2012 (X7900 + Samsung SSD) and I've experienced no problems (24/7 in use). Don't expect to much advantage: benchmark of the T7300 is '1199' while the X7900 reaches '1704'.
I'm not for sure the types you're mentioning are socket 478 one's. This is a limmiting factor in the 7.1 iMac of course ... Otherwise: if the processors dissipated power is about 35-45 Watt I would give it a try ! Frank.
Also upgraded an 20" iMac 7.1 mid-2007 from a T7700 to a X7900 processor without any problem. Changed the original WD 320GB harddisk to a Samsung 830 SSD (128GB). Did'nt test benchmarks, but the system seems to work remarkable faster & because of the SSD the system becomes extremely quiet !