On the Daystar upgrades, my experience has been that the 25MHz models up-clock to 40 MHz without problems. I didn't even need to swap out the 68040 for a faster model. I think I usually added a heat sink to the 68040. I don't know if the Sonnet has similar head room though. Daystar products were the best in their day.

That doesn't look so much like a NuBus upgrade as an LC slot upgrade to me, although it's hard to tell from the photo. I'd do a little more research before plugging it in and powering it up.