Not the OP here, but, aside from my x86 systems, I have several RPis crunching away on OPN: one Pi 3 and a 3+, one Pi 4, and one Pi 400.

My Pi 4, and all Pi 400s, are 4 GB RAM (there are 8 GB Pi 4 and Pi 4 Compute Modules available). The Pi 3s are 1 GB each, with zram replacing swap, and a Pi 3 has no problems keeping four OPN tasks running with zram. That being said, the 4 and 400 are much better bets if you haven't bought any Raspberry Pi hardware yet. I see an estimated 9.8 hours for an OPN work unit on the Pi 4, and 8.1 on the 400. Older Pis take far longer.

I just put my Pi 400 in service yesterday, so I have yet to get a RAC for it, but I'm getting mid-700s on the Pi 4 with a cooling fan, so I could probably expect high 800s/low 900s for the Pi 400. I got the 400 for the purpose of retiring an aged ThinkPad, and I think I'll be picking another one up to replace the other ThinkPad as well. The laptops draw 42 watts apiece running full-tilt yet heavily throttled, compared to 7 W for a Pi 400 (with no throttling whatsoever). I'm better off using the ThinkPads as laptops than as crunchers.

The per-thread BOINC benchmarks I got for my 400: 2524 floating point, 68987 integer. Looking over my spreadsheet, the 400 is my absolute aggregate MIPS per watt king, regardless of whether it's integer or floating point.