Frankk, I'm not sure why Lee would fit your "custom" stem poorly to your bullet, unless it's not really "custom" or they just did a rush job on it. I haven't had Lee do that for me, but I've done some myself (one of the many advantages of having a lathe); I match the angle on the bullet nose and end up with a stem that fits pretty snug. That does help with concentricity; even the Lee seater is capable of good straight seating when the stem fits the bullet well.
With that said, you'll like those Forster dies a lot more of course. They are nice equipment for sure.
If you're willing to do it, it would be interesting to see runout results between your Lee die with either seating stem, and the Forster die. I'm sure the Forster will do better, but you could quantify how much, and whether the "custom" Lee stem made any difference.
