Powerboost and other burstable WAN links

[xcrustwadx]

My ISP (comcast) allows bursting (powerboost) over the specified bandwidth cap.  For instance I have a 8Mb/768kbit connection but the powerboost will allow bursts for the 1st 10mb of a file to 16Mb/2Mb and then throttle back to 8Mb/768Kbit.  My question is does traffic shaper only "kick in" when the link is saturated or will it always put higher priority traffic before lower?

In my case the penalty upstream queue is capped(upperlimit) at 400Kbit.  Then I set my max upstream to reflect the "burstable" upstream ~ 2Mb.  Then i fill the penalty upstream queue to its 400kbit upperlimit.  If i start a higher priority upload will it still take the higher priority traffic before the lower(penalty upstream) or will this only happen when traffic shaper "sees" that the link(2Mb) is saturated?

I hope this is clear.  I want to take advantage of powerboost with a traffic shaper in place.

Thanks.