Yes, windscreens should be very carefully used on toppers. Anything that will potentially hold or conduct heat to the canister can be bad with no control system in place to regulate that heat cunducted to heat sensitive components. The whole thing is a bit kludgy in my book. Toppers are, perhaps, one of the lightest stoves out there, but they are also prone to user modifications.
The external stoves, however, do not suffer from this. With any type of adequate air flow, only the fuel line would be vulnerable to heating/potential damage, even at relatively high heats (~200C or so.) This is far safer, it removes the fuel from the heating area. It is also heavier to build such a stove, as we know...
Anyway, things do not add up to me. Canisters of mixed fuels do NOT necessarily HAVE to have the super high pressure containment Roger so casually dismissed.
Propane is propane and has a gas pressure. Using pure propane is obviously dangerous in the light duty canisters used for camping fuels. Why doesn't the addition of 15-30% propane cause a dangerous situation just as 100% propane does? Acetylene has a very high gas pressure, yet we use it all the time as cutting/brazing torches at quite a bit lower pressure than has been indicated. This is because it is disolved in benzene or similar materials reducing the internal tank gas pressure. The same way that isobutane (or butane) reduces the internal gas pressure for our canisters...not that I am advocating acetylene as a prefered camping fuel, I am just saying there are facts we are missing when we talk about canister fuels, in general. The boiling point of the mix is also lowered. For example, freezing ethynol and water will drop the freezing point considerably below the freezing point of water, alone. 'Corse we are not interested in freezing points, but it also reduces the boiling point as any moonshiner will tell you. Adding ~25% propane to butane causes a reduction in overall canister pressure compared with 100% propane. The form a system at any temperature/pressure.
As far as HE pots effecting the flame pettern on the 300t, no doubt it will. This is relativly unimportant. What IS important is the mixing of sufficient air to cause complete combustion (low CO) and purging exhaust gasses(and heat) from the flame. Since a fairly high heat is required to produce CO, soaking the heat up into the "fins" would further reduce CO production. This would be facilitated by the HE pots, so I tend to think nothing would happen to the flame even if it did change shape, and, you would simply realize the better efficiency of the heat exchange.