Disclaimer: The following is just my take on things.
The progession you experience in shoot'em ups beyond the 3 week point is quite fascinating.
You're gonna laugh at this quick crappy sketch:

^This is basically how progression works for me on all shooters. I talked it over with Plasmo on the way down to Stunfest and he experiences the same thing.
Basically it goes like this:
* First week on the game = rapid progression.
* Always followed by a short period of Regression. Strange but true.
* Followed by more rapid progression until the 3 week point.
* But then you hit what you may feel is a plateau or a brick wall. Beyond 3 weeks the progression is slow, very slow, and steady. Progression on this slight gradient is measured in months not weeks.
The Japanese know this and they plough through it by putting in 10 hour days, thereby shortening the months.
This slight progression gradient after the 3 week point is interesting to me. This is where your brain is accommodating all the patterns in the game and slowly ironing-out all the creases. There are basically no spikes in general skill during this slight gradient. There can be jumps in score due to new techniques learned from replays or your own experimentation, but no general skill jumps. This increases slowly and steadily.
* Somewhere along the slight gradient line may lie what I call the 'pain barrier'. It could lie at 1 year or 2, but its there.....lurking.
ArrogantBastard no doubt encountered this pain barrier during his 5 year battle with Progear (Great Score by the way!).
I might redo the crappy sketch & update...
Ps. I left the HELLO! in there because it completes the Malibu Barbie toytown feel of the sketch
