When fitted for the AP3, the 31 degree 7 iron AP3 was a little longer and higher launching than the 30 degree 7 iron AP1. I can't say this would be everyone's experience, but if you were to see similar numbers, I wouldn't bother mixing AP1 and AP3. If it was AP2 and AP3, and the 3 was easier to hit, I'd consider mixing that setup.

I did a mixed set with the first generation AP series. It worked real well.

However, recently I tried to do the same with a mix of 718AP1, 2 or 3 and found it much more difficult because of launch/loft differences. I had problems with distances and launch that could not be easily resolved. I'm sure it could be done with the help of a good fitter and some extended range time. I just decided it wasn't worth the effort.

The loft differences between the AP1 and AP3 4 through 9 irons is 1 degree, with the AP1 irons being a degree stronger. A 1 degree change in loft generally translates into about 2 yards of carry. I know I am not good enough of a ball striker for it to make a difference, ymmv.

The AP1 4 and 5 irons are hollow head irons. If you can find a Titleist Thursday, they should have a full set to try.

FWIW, I have 6-GW AP3, and, I had thoughts of adding a 5 iron later, either AP1 or AP3, but, I am sticking with my 26 degree I25 hybrid for now.

I went with 4-7AP1, 8-48* AP3 and couldn’t be happier. My club fitter bent the AP1 to match the lofts as if it was a full set of AP3.