Gary I had two old pilots look at your description of your aircraft function at altitude--one is also a 30 year FAA small engine inspector/advisor. You said your plane felt like someone was pulling on the tail when you switched off a Mag at altitude. You should have been at optimum lean at that stage and the drop should have been almost un-noticed.
They said when you pre flight you should run up to rpm (at full rich) and switch Mags without more than a 50 rpm drop. The twin system is for safety not power. Not my thoughts but theirs based on your words--get your plugs checked and be safe.
George Camp