First, any suggestion to modify your PFD should be viewed in the light that will probably "invalidate" that PFD in the eyes of the CoastGuard, insurance company, and manufacturer warranty department.
Second, padding is placed where it is to control the floating orientation of the PDF. In the olden days, some PFD's would keep you afloat but would allow your to float with your face in the water. I'm sure today's designs keep your face up and out of the water. Modifying the padding ratio could interfere with that design.
Third, long ago, when I had a problem, I discovered it was useful to look around a little. Was the problem with the thing/object I was dealing with, or was the problem with me. If I saw that many other people could do what I wanted to do with what I had, then I knew I had to work on myself rather than change what I had. For example, I am only 5'6" and paddle a Mariner which has a high back deck (especially if you are only 5'6") and I use a MisFit PFD. And when I do it just right, I roll right up - no strain, no effort (I'm not saying that happens often).
To be fair, I was trained to do a sweep roll that primarily uses a forward lean - though finishing with a back deck lean can sometimes "save" the roll. And I'm not saying you should stick with what you have when there is a better solution for you out there. I'm just concerned about modifying what you have as the modification could reduce its effectiveness when you need it.
Last, I would not be a fan of "inflatable" for such a basic piece of survival gear. I'd want it as simple as possible - no moving parts, no switches, pulls, CO2 canisters, etc. to possibly fail.
All that said, my other PFD is an Astral Air Escape - designed for "high back" kayaks. It has some padding up by my head, but not none (or very little) by my mid back and lower - so I can bend backwards easier. If I recall, it wasn't very expensive and was sold by REI - which means several times of the year it goes on sale at 20% off or you could use the "buy $100 worth of stuff between X and Y dates and get a $20 gift credit" (with must be used within a few weeks) option to lower the cost.
I just Googled the Air Escape PFD and didn't see a ready link - so it might be discontinued. But if Astral made one, they probably make another with a similar style for high back boats (or people who what to lay back roll). They are out there and if you find one, you will have it for a long time. So divide the cost by a number of years (and if necessary the number of months) you will have it and the cost per year (month, day) will probably be worth it for something you can count on and works with your style.
This is a much longer reply than intended but was triggered when I read the words about modifying the original design of a PFD to accommodate and issue that was solvable by other means.