A lot more? No way, and I just read the specs and the service manual. Based on the info in the service manual, I would say the RX-A1080's power supply is comparable to that of the Sansui amp. For one thing, the AVR has 20% more power supply storage capacitance. The Sanui may do a little better in terms of "continuous" rating, but for real world use, the AVR would do better for sure, especially in terms of dynamic output.
AVRs are not rated for 4 ohms, but for two channel use, the power supply should have no trouble outputting more than it would into 8 ohms. In this case, it would do as good or better than that Sanui.
For two channel into 4 ohm, you can see the actual measured output of the RX-A1080 if you follow the links below to two different bench test sites:
https://audiovision.de/yamaha-rx-a1080-test/ (See that it measured 214 W into 4 ohms, 2 channel driven)
This is a review and detailed measurements of Yamaha RX-A1080 Home Theater Audio/Video Receiver (AVR). I purchased this solely for the purpose of testing through my company (Madrona Digital is a dealer for Yamaha among many other brands). The RX-A1080 is part of the upper tier of Yamaha AVRs...
www.audiosciencereview.com
In the ASR review, it measured only 133 W into 4 ohms, but it was at about 0.01% THD+N (SINAD 79), whereas Audiovision.de, being a German site, likely based theirs on DIN, i.e. 1%.
The Sanui's output spec is 100 W into 8 ohms, and only 120 W into 4 ohms, no THD+N spec given, though for the 100 W 8 ohm rating, it does say 20-20,000 Hz at no more than 0.005% THD, even that is very clear. Typically such spec should say something like 20-20,000 Hz, 0.XXX % THD+N at rated output into X ohms.
Also, the 0.005%, while quite good, but it does not include noise. You can have low THD, but if noise is excessively high and is the dominant one then it would still be no good. Being 30 years old, the Sansui would likely not do as well in terms of THD+N.
In your system, I would rather use the AVR for the 4 ohm infinity speakers and let the Sansui drive the center speaker. In theory, the Yamaha should give you cleaner (lower THD+N) among other things because the signal path is simpler. The Sansui does not have input for the Yamaha pre-outs so you have to use the line input, and that means the signal has to go through the preamp/volume control of the integrated amp, no gain there, only deterioration.
People tend to think those old/near vintage amps are more powerful than AVR just because they are heavy and are "integrated amps". That is not true, like cars, older heavy body on frame cars used to do better in crash dummy tests, not the case any more. Instead of trusting the old hearsay and myths, it is better to trust technologies that actually do advance with time, just my two cents, ymmv..