Well first of all we don't know for sure if anything is half clocked(but as dresdenboy found out,there are good indicators). Second, why do you think 4 flexfp can work at 3Ghz within 125W(if they are indeed "half clocked")?
As for L2 cache,if it's half clocked then bringing it to full core speed could potentially increase performance quite significantly in some cases. L2 is what matters for desktop workloads while L3 is what matters for server workloads(due to data set size differences).
We have to wait for trinity and see what it can do without L3. If it is 10% or so faster than Bdver1 at same clock,then Vishera(with L3) could be regarded almost as real next gen Bulldozer since it would bring larger speedup versus Bdver1 than Bdver1 brought over Thuban (on desktop).