To answer one of your questions, no you would not want to us 15% Oxygen, the normal recommendation for Ar/O2 is 1 to 8%.
I worked with PGMAW of steel with 8 to 10% CO2 for a number of years. To be honest, I started out using a pulsed spray mode and did not attempt non-pulsed spray, I already had pulsed so I used it.
For the question of spatter, in my experience with relatively thin parts, around 1/8" thickness, I had to run a rather tight arc/low voltage to get good consistent fusion and bead appearance, particularly where the joint was narrow and deep. With the tight arc you get some spatter, but thats was the price for a good weld. In some cases the material thickness and joint would allow an increase in amperage and voltage, and I could get good fusion with the more traditional spatterless spray associated with longer arc length.
If you can wade through a ton of opinionated commentary, you might find some good info from Ed Craig. From what little I've gathered, he claims that PGMAW is a waste, a consiracy of the manufactures to sell new machines, and that anything suitable for GMAW need can be done non-pulsed. This may be true for heavy section flat position welding, but from personal experience with PGMAW in orbital pipe welding, other out-of-position welding, and thin section welding, PGMAW can accomplish welds that are not possible by conventional spray GMAW.
A good source for info on GMAW gases can be found in this AWS article, GMAW Best Practices, Feb. 2006 WJ.
http://files.aws.org/wj/2006/02/wj200602/wj200602-46.pdf