I am not sure that "more accurate" is correct. That is an age old argument that will not be solved here! There are credible people on all sides of the issue that claim this or that dyno is more accurate.
What is important is the delta and the relationship from one machine to another. The gains seen on that dyno-pack correlate fairly well with what was expected based on the dyno-jet results.
Everyone must realize that the dyno-pack may not be the most realistic measurement of your car as it requires you to remove your wheels - and last time I checked, we need those wheels to drive the car on the street or track!
Also, when you are talking about a modification that recaptures lost drive-line efficiency, removing one part of that drive-train will lesson the gains to some degree.
By far the most accurate way to test motor gains is with an engine dyno. However, with mods like pulleys, flywheels, CF drive-shafts, differentials, lighter rotors, lighter wheels (anything that works to recapture efficiencies lost in the drive-line), you must use a chassis dyno (and as it shoudl be becoming clear, they all have pro's and con's).
We use a DynoJet 248c as it is the single most commonly used dyno in the US. You can find one in nearly any town, and you will be able to see gains within 1-3% of advertised on any of these machines anywhere. This allows us the opportunity to have customers see the gains that are sold with the product. Is this the "most accurate" - dunno. Again, that is a bigger argument, and one that will not be solved here. However, we feel it is better then using a dyno that few have access to (like the Maha, which is what a leading MBZ tuner uses to show gains).
So net/net, it does not matter which tool you use to measure. You just need to see the before and after and realize how that correlates to the delta from another machine. There are at least half-dozen commonly used dyno's (dynojet, dyno-pack, mustang, maha, etc) - and they ALL read different (arghhh!).
I agree that the low-end gains are the most critical as that is where you will feel the biggest improvement - from the stoplight or on the initial application of the throttle.
PS - PencilGeeks test was done in cooperation with evosport. He and I had communicated and I had asked him to do an independent test. Just for full disclosure!