My reloading stuff is still packed away, otherwise I could do some quick cold weather tests with HVR. Someday ......
Now that I have thought about it some more, I have sometimes seen a trend for the first shot velocity to be highest, and subsequent shots losing velocity. That was usually a sign of fouling, and I have seen the trend with a variety of lubes. But, unlike your situation, what I was seeing was "normal" velocities in a clean barrel and "below normal" velocities as the barrel fouled and the gas seal went to crap. I don't recall seeing "above normal" velocities with cast bullets. To the contrary, it is usually a struggle to match loadbook velocities for a given powder charge.
Automotive lube ? It's been tried by many, no one has reported exceptional results with it. It is often used in homemade lubes for plinking-type loads, but you don't see these homemade lubes setting new world records in CBA matches.
If a bullet lube's job was merely to lubricate, we'd all be shooting Mobil 1.
A cast friendly-gun is not sensitive to lube. My Marlin 357 scarcely cares what lube is used. My Ruger Speed Six 357 shoots well with any lube, or even no lube, but gives higher velocity, lower standard deviation, and slightly better accuracy with hard lubes.
The most dramatic change due to lube that I have seen was in a M700 30-06 with a 165 gr. "LBT" style bullet. Switching to HVR could pick up 200 fps. A 180 gr. loverin was not nearly as lube sensitive, though it still favored HVR. I think the loverin's design creates an inherently better gas seal, so it is less dependent on a lube "bandaid."
Get a copy of The Fouling Shot published by the Cast Bullet Association. I'm not a huge fan of the CBA, but one useful thing they do is publish load data for their matches, including the lube used. HVR is popular, along with Blue Angel and Tom Grey's lube. The harder lubes seem to be gaining popularity amongst the target crowd, while the traditional soft lubes and homemade lubes seem to be losing ground.
As for HVR causing pressure to spike, I am very skeptical. Recall my test where I swabbed a barrel with valve grinding compound ???? The valve grinding compound didn't do squat to pressure or velocity. I think Rooster boosts velocity and pressure by reducing gas leakage, but I find it hard to believe that HVR would create more friction than valve grinding compound !!!!!
A technician at a powder company once told me that they had privately tested Lil Gun (interesting how they keep tabs on the competition) and gotten very erratic results. I no longer remember which company he was with, and anyway, he didn't want to start a pissing match with Hodgdon. I have found Lil Gun to be a great powder for small rifle cartridges -- though no better than H110/WW296 -- but inferior in short barreled revolvers.