After holding down A and C your meter in the upper right goes from blue to red. If you fire holding down A it steadily decreases.
Does A+C act as a toggle into red mode ? Or do you have to keep them pressed ?
You have to keep them pressed.
Notes from the field:
High scores at Hey for Power mode were 230 bil (with B type). Alpha channel had a 210 bil clear with Strong. I scored 160 today but still haven't made significant inroads into stage 5.
The guy who hit 230 was an absolute beast at the game already, flipping in and out of hyper mode, hitting every white bee and showing me some new tricks.
Like: bomb and life items cancel all bullets on the screen, so he would kill a bomb carrier, let the bomb item float around and then grab it when he had the boss shooting out a ton of shit with RED.
People have some crazy patterns already to maximize on the bullet canceling properties of the bees. For instance the first installation after the stage 3 midboss, immediately after destroying it, dive directly up and take the yellow bee as the helicopters come down. Lots of stars. All kinds of stunting today.

Because chains usually don't get cut after dying in Power mode, I think this will be the first mode to really hit the score ceiling in a month or two. The game is harder than 1.5, but that's not saying much since 1.5 is really easy. However scoring on 1.5 in general is, at this stage, much much harder than BL. It will remain so because of all the anal chaining and obscure hyper rules that you have to memorize to carry your chain across the stage.
The real challenges will be Bomb and Strong mode, for whom getting into the Ura boss route is already quite tough since hypers are slower to build, and the more you use RED shot, the more bullets fill the screen. I saw people getting the regular boss route over and over again.