I'm currently setting up my new truck and did the same test after reading about it here, couldn't believe my eyes at how easy it was to get excessive belt slack from such little pressure on the shock towers.
I have a B44.1 saddle pack setup on the truck and I think the chassis flex is a lot worse without the stick pack brace/bracket. It flexes so easily that I wouldn't even run the truck until I made a chassis brace, which I did today. Looks like a chassis brace it's going to be mandatory for a saddle pack setup IMO.

I also noticed (as mentioned above) excessive belt tension really torques on the Topshaft and slightly twists the transmission, which in turn will affect gear mesh. I would be surprised if Associated does not include a chassis brace on a Factory Team kit in the future. This truck really needs one IMO.

I made the brace from some solid aluminum rod I had on hand and Rocket City ball end that are commonly used to make Clodbuster tubular suspension links. The hardest part was drilling and tapping the ends for the 6/32 studs to go into.

I had to keep the link level and up high enough so I can remove to saddled packs without removing the brace. I was concerned with the rear attachment point possibly breaking so I reinforced it with a large colored washer underneath and a locknut to spread out the load on the plastic over the motor. With the Tekin Pro 4 540 motor there is plenty of room above it to allow for some type of reinforcement piece add you can see in the last photo.

Now the chassis with saddle packs passes the chassis flex test Craig was trying above.