Honda what are you measuring to come up with that 8-10 degrees? Is the oil boiler side of the HX circulating at the same time the OWB side is? Assuming just a short loop through your fan box and back or is there a bypass on the A/W exchanger or what? If they are both circulating you will catch up at some point right? Return temps will get higher and higher every exchange under no load until you are matched.
Woodman, storage and buffer tanks are the same thing- it's how you choose to use them that differentiates. Potato/potato. That's neither here nor there though. If you want constant circulation through your coil, as you said this will limit stratification by drawing through the tank 24/7 so a secondary tank with a shunt pump to keep mixed is what you're after, right? That way when the entirety of tank #2 gets below temp, it will draw hot water from stratified tank #1, and that replacement water will be reheated by your plate HX from the OWB loop. Am I on the right page now?
As for the plate, the short answer is yes- bigger is better. A 5x12 HX with 20 plates let's say will pick up 60 degrees at 5 GPM. Now that same 5x12 with 40 plates, you may be able to pick up 100 degrees with all other things equal. There comes a point where flow becomes a factor and you may need to go to a bigger plate (like 10x20) for port sizing to keep X amount of flow, but residentially that's all but unheard of. You need to know the GPM, heat load, and desired temp on both sides of the plate to be able to properly size an exchanger. As I said before, it's entirely possible to match temps on both sides but it depends completely on the aforementioned variables to size the plate accordingly. For what I think you have going on, assuming 7 GPM on the OWB loop and 7 GPM through your A/W exchanger, I would assume you'll want to minimize recharge time for the storage/buffer tanks. Matching temps with equal flow on both sides is going to require a BIG plate- like an 80 or 100 plate 5x12 or a 20 plate 10x20. You can downsize a little bit by reducing flow through your A/W coil to say, 4 GPM which will help recovery time across the plate but to minimize recharge time go big. Keep in mind this is going to pull your OWB down like a freight train during the entire recharge cycle, so hopefully it's got a full firebox at recharge time