The NSS cores can offload PPPoE traffic, but if that's enabled enabled in those OpenWrt based builds is another question.
Neither irqbalance nor packet steering is going to make a material difference, the r7800's ARMv7 cores won't do more than that - not for routing plain ethernet (at most 550-650 MBit/s), even less for PPPoE encapsulated traffic. Software flow-offloading might push the envelope a little (less for PPPoE, than it would for plain ethernet), but that's not without quirks either - this hardware is just designed with NSS offloading to a proprietary firmware running on its two 800 MHz little-endian ubicom32 derived cores (which lay dormant on OpenWrt) in mind, without that (as on OpenWrt) it won't reach those speeds.
Faster hardware, such as x86_64, is the better option for this use case and the following might provide some ideas (more in the later parts of that thread):
Again, PPPoE and sqm (should you need it) are demanding, so even those AMD Jaguar cores or baytrail might (PPPoE) or will not (sqm) suffice for full line speed at 1 GBit/s, but something starting around the ivy-bridge based c1037u will (but haswell and newer would be more power efficient, depending on the mainboard).
Personally I have gone this route, and I'm not looking back - the r7800 can still make a fine AP.