I know this thread is a little old (lol), but I thought another reason for loss in rail was due to higher rpms. I've had a lift pump since before I had efi live, the top end has always seemed a little week...kinda like it couldn't keep up. I logged it and even took some video doing 1/4mile 4wd boosted launches at wot to watch my rail drop. I've got 320k km on my truck now. My 150 tune had the reassure drop but it wasn't as noticeable as the 230 tune is now. I found that among common problems (such as a weak fprv), the pump itself seemed to "defuel" at high rpms causing a drop in pressure. I'll have to find the thread again that talked about it, but it had me looking at exergy's sportsman cp3 which i believe was just a modified stock cp3 that would hold pressure at higher rpms rather then allowing pressure to drop.
It s pretty common knowledge now that you need a pump modded to fuel properly over 3,000 rpm. The most popular seems to be exergys sportsman cp3...