Can't tell you anything about Teknic bags, but I'll relate my experience with an RKA magnetic bag. My RKA bag has 10 magnets, three on each side on the flaps that fold down to about where the Ducati label is on the side of the tank, and two in the middle at the top of the tank. The magnets are big and very strong, being about two inches in diameter. It also has some kind of surface that goes against the tank that doesn't slide.
That tank bag stays where I put it with no movement at all, so I don't feel the need to have any additional straps. Some other tank bags have fewer and smaller magenets and I wouldn't trust them without a strap in case they come loose.
Sorry I couldn't answer your question, but at least you now have a reference point.