I've got a friend who uses the old 8" (7 & 1/4") BBS coil on his Excal and he does very well. Swinging my GT in the water the stock 10" Tornado is perfectly fine to me resistance wise even near shore with wave action moving a current in and out. I found the 15x12 was too much drag for my tastes in the water and wore my wrist out. Always seemed to be trying to force it to swing a little faster than the resistance would allow me to push. The 12x10 for sure is more resistance in the water than the Tornado. Probably why the S-12 seems more popular among guys using aftermarket coils in the water, as it ain't squarish in shape like an SEF.
Far as hunting the dry sand the 12x10 without a coil cover is lighter than the S-12. I think from memory the 15x12 without a coil cover is lighter than the 10" Tornado with a coil cover (weights are in the coil chart in the coil sticky).
If I was buying an Excal my personal choice would be the 10" Tornado on it, just because that 8" Tornado is so tiny at only 7 & 1/4" in true size, but like I said my friend does very well with his on his Excal. If your beaches are heavily mineralized or contain a lot of iron then the 8" would probably be the better choice, not just to see between the iron better but also because in some mineralized soils or sands a smaller coil will see deeper than a larger one due to a larger one sucking up too much ground matrix. My 15x12 didn't get as deep on coin/ring sized targets in my soil than the stock 10" coil, but it did get deeper in my mineralized beaches for some odd reason.
So far as your beach/water. if there is a ton of iron around or the sand is badly mineralized I'd lean towards the 8" coil, but if you are one who likes to move and cover ground faster, or hunting where a bigger coil should give you more depth, then lean towards the 10". I personaly couldn't stand trying to grid the water with a tiny coil not being able to see the coil to insure I wasn't missing spots during my grid, so that's one of the reasons why if I was to buy an Excal I'd get one with the 10" on it.