I think KC3's post sort of supports a generally accepted fact that boosting the performance of the "B" / "HGT" engine is not that easy/advantageous unless you go very radical will full race stage "n" options and even then compared to other engines the gains are less rewarding.
I've never researched or tried to understand why performance gains can be so limited. Certainly the Hitachi ECU is a limiting factor as there are next to no mapping/tweeks that can be applied to this ECU (as far as I'm aware/understand).
Sometimes an engine "package" comes out just about perfect and their is little gain to be obtained without radical radical changes. Not just cam shafts, but bore & stroke, etc. Almost a new engine design.
I think / am led to believe that the "B"/"HGT" engine is about as good as it gets. If new cams, and at what expense, only buy buy 5 BHP (3.8%) increase in BHP one also has to consider what torque profile may have be lost/sacrificed in the process.
As with all tuning improvements there is that "window of opportunity". By that I mean if you are already looking at an engine rebuild then the labour costs are effectively written off as you can't avoid them and the small extra cost for different cams, etc. acceptable and potentially worth the step/cost.