I don't think grip is lacking only on the front side. I posted a quote of Webber in the Bahrein topic where he say engine power is unusable because of the limit of grip of the BS tyres.
Yes, he doesn't drive an R27 but his saying were aplicable to what I've seen of on board of fisi and heikki, espcially after the last turn, when we hear TC kicking in for a long time.
Also, unlike many components that could be tuned to adapt to the bridges, engines are those of last years, everyone stated that "only a few modifications were possible". So for sure Engine is the component that hasn't been designed for this year. Renault are famous to build atmos engines very torquy, in a wide range, more than high-revving blocks like manchild reminded recently.
Which lead me to an hypothesis: 2007 bridgestones are favorising upper range torque engine philosophy, and makes engines like RS27 (which design, I insist, has been thought in 2005-2006 as part of a perfect package with Michelins tyres) inefficient in regard to the grip available.