How many rounds do your 10mm SS mags hold? I see their 9 round mags advertised but I never saw 10 rounders. I've been shopping for 10 round 10mm/40 single stack mags haven't been able to find any.
I was just looking at my McCormick/Shooting Star mags and they used a low profile pad like the one on the Power Mag and lengthened the tube by .4 or less, the OAL would be about the same as a standard Shooting Star with a basepad.
A 40 single stack would seem to be the ticket for starting out in Limited 10 if the mags were available. Everybody I talk to with ties to the industry doesn't seem to think there's enough market to justify making them though.
I tried this once with mixed results. Get a 10 rd .45 mag and load it with .40's. Should hold eleven, mine did. See how it works in your pistol. Mine is just inconsistent enough to not try in a match. I would go with the 9 rd mags in a match environment at the moment. It's worth looking at.
You can get 11 in a Wilson and Mecgar. The Metal Form will hold 12. The Metal Form mag can be cut down to hold 10 and still fit in the IDPA box. It is slightly longer than the typical add on mag well. The Metal Form .45 mags have a longer lip profile which retains the .40 bullets better.