Rawhide, I've been using bismuth shot for waterfowl, but I've been interested in steel shot in muzzleloaders for a long time, so I've read everything I could find from others shooting it and have done some experimenting of my own. I've come to the conclusion that my 16ga double is rather lightly built for success with steel, but I can summarize what I've gleaned from my reading.
I don't know your background, so please forgive me if any of this is belaboring what you already know.
First, as you've provably read, it is much easier to have success with denser shot at muzzleloading velocities, and you can use bismuth, Nice Shot (Ecotungsten), and (according to several people) ITX Original 10 without any special shot protection. Unfortunately these are all much more expensive than steel shot.
Assuming that it fits in your 11ga bore, you can try the SP10 wad, but it is intended for lead shot. Since you're not shooting through a forcing cone or choke, the SP10 might be sufficient by itself to use with steel shot, but I'd strongly recommend putting a Mylar wrap inside it for additional protection. Most shooters use some sort of conventional card and/or fiber/felt wads under the plastic wad to keep the powder gasses away from the plastic, and some will cut the cushion section off the plastic wad and just use the shot cup over the over-powder wad(s). Given the design of the SP10, it looks be difficult to trim the cushion section off with consistentcy, so it might work better whole. Most shooters use some sort of lubrication, whether swabbed in, applied on/in wads, globbed between wads, or globbed into the cushion section of the plastic wad.
I'd estimate that the SP10 would hold 1.1/8oz to 1.1/4oz of steel shot, depending on the size and whether the wrap is used. Most guys reporting consistent success shooting ducks with steel in muzzleloaders seem to to be using #1-#3 size, and using more powder by volume than the equal-volume load for the same weight of lead shot, maybe 1/4 more (or more). For example, with a 1.1/4oz lead-shot load, the same volume of powder is about 3dr/82gr, so 1/4 more would be 3.3/4dr/103gr, which happens to equal the "heavy field load" for a choked 12ga. With conventional loads in unchoked muzzleloaders, these heavy powder charges would tend to open or completely blow the patterns, but the plastic shotcups, and especially the stout steel shotcups, tend to tighten the patterns and compensate for the heavy charges. FYI, a volume of powder equal to the steel shot gives 3/7 to 2/3 more (depending on shot size and measure diameter) than the lead-load equal volume, which would be very stout loads of 117-137gr.
Naturally, as with all things in muzzleloading, you'll have to experiment to find what load gives the best combination of pattern and penetration in your gun with your components and loading techniques, and whether it works for your intended usage.
Good luck with all this.
Regards,
Joel