I haven't driven this mod since I first signed up, but if memory serves me right, they do auto-blip on the downshifts.
To get the optimal result you need to blip the throttle. Still you can not do it but then Autoclutch needs to be ON and you need to have high Brake bias to the front and high coast. But i recommend that you blip like with every GT car.
Also check in Userdata - Your name - PLR file and search for Auto blip and check if its on 1.
I rarely blip the throttle on downshifts. My work area is also my cockpit, so I generally find myself using the paddles to shift, being that it's just to cramped on my desk with the 6-speed shifter. Using the clutch and heal-toe downshifts just doesn't feel right to me when using the paddle shifters, so I run auto-clutch most of the time. I find it's really just a matter of proper brake balance/pressure and being smooth on your downshifts, particularly in heavy braking areas. Some cars/mods still require a little manual blip on downshifts, but most can be driven just fine with just the auto-clutch selected.Main thing is to have Auto Clutch on, if you find it unstable. I use it also but only in GT cars because its like you said better for under braking and keeps it more stable.
But i still cant downshift without blipping it, even with Auto Clutch on.