4 speed ka-booom !

Yeah if you're shifting at the top of the power curve I think you may not be trapping as well as you could be. But if your combo hits a wall at 5900, I could be wrong.
My combo looks kinda like the one in this link http://www.hughesengines.com/Upload/TechArticles/land_dyno.jpg Notice the shape of the curve,off to the right. I can tell you that the power does not drop off very fast up there but continues at least another 800 rpm or more. So if my powerband,dictated by the tranny, was 71%(which is the approximate split with the 2.66box), and I shifted at 5800(the approximate power peak), I would be dropping in at 4118rpm, where it comes in at something like 330hp. BUT, if I carry the rpm up to 6800, where power might be 400, I would drop in at 4828, where the hp looks to be just a tick under 400hp.What this means is that, even though the car may be slowing down at 6800, it will be dropping in at 70 hp more, in the next higher gear, where it really needs the boost. This is putting more average hp down during the run.
Now let's say your tranny had tighter ratios, like 78%. With this ratio, I could outshift at 6500/410 hp, and come in at 5070/410hp, thats even more average hp during the run. And the engine will appreciate the 300rpm lower shift point.
But let's say your engine hits a wall at 5900. And the hp falls off rather steeply thereafter. Without a dyno sheet, the optimum shift point can only be determined empirically,by running shift loops, and comparing mph numbers.The dyno sheet just gets you there a lil quicker.
Of course having the optimum rear gear, ensures trapping at the best point in the hp curve as well.This is a different point from the shift point.
I read that time slips don't seem to show an improvement. I hope you're keeping records, Cuz by looking at the mph numbers, you can get an idea of the average hp that she's putting down, using different shift points.The highest mph numbers, with a given rear gear, are indicating the highest average hp being put down.
There is a short-cut I have used. It involves those "stick-onto-the-window" accelerometers. This device purports to able to generate a HP chassis curve. Well I use mine for comparative purposes. The graph it generates is pretty small, but the data is useful. If you transfer the data to 8.5 x 11 graphpaper, you can see the shape of the power curve. By then using a straightedge, spanning the rising and falling sides of the curve, and adjusting the height of that straightedge, to generate a line,equal to your powerband, it will spit out your shiftpoint. It's pretty neat.
If you afterwards make changes to the engine, such as an intake, or a cam, you just generate a new graph, and the new shift rpm pops out. But, byNlarge, the shift point is dictated by the tranny ratio spreads. I love that tool. It saves tons of wear and tear on the engine,and tracktime, doing shift loops.That means more time having fun.
Be forwarned that tool is just a tool, and some shift loops do need to be run, eventually,to verify that the chosen points are in fact optimum.