Microwave more than one container at a time

1100 Watt means 1100 Joules per second (energy over time). 1100 Watt over a period of 240 seconds therefore is 264000 Joules. To deliver 264000 Joules of energy with only 800 Watt takes 330 seconds (5.5 minutes), as you expected.

As KatieK noticed, there are some additional concerns. A good recipe will tell you to let the cooked product stand for a while. This allows heat diffusion, so all those Joules of heat will be distributed well. But if you cook at lower power, then the heat will already be diffused more. I.e. at lower power, you don't need to rest the product as long.

Another difference might be that you're not just heating the product, but you're relying on a secondary effect such as killing germs. For that, you'll need to have the entire product above a critical temperature for a certain time. This in general means that you don't need to adjust the times as much as you'd expect by the simple formula.

On the other hand, a warm product will lose heat to the environment over time. I.e. not all of those 264000 Joules will stay in the product you're heating. And with more time spent at medium temperatures, there's more heat loss in a low-powered microwave.

My kitchen manager (fine dining restaurant) said that that the only thing a microwave should be used for in a professional kitchen is to heat water.

Microwaving tends to dry out the outside of vegetables, and hurts both texture and flavor. Proper blanching takes about 5 minutes once you have water at a boil, and maintains both the crisp, fresh flavor and full texture. Blanching also tends maintain color better, because the outside of the vegetables heats to the same extent as the inside, and this is doubly true if you use an ice bath to cool vegetables after blanching.

So yes, you are Doing Things Wrong. That said, we all take shortcuts at times, and microwave ovens do work rather well for thawing frozen vegetables. With frozen vegetables you've already lost a lot of the texture, so the microwaving doesn't do much further harm.