So through my own research it seems that an i9 9900k is the max that can be put into a 19,1. I plan to replace the fusion drive with an SSD this summer and I was thinking of just digging a bit deeper and swapping out the CPU. Now I just have to decide if it's worth the cost doing so. A quality 2TB SSD and an i9 would put me around $700. I typically keep my Macs for a long time, but I'm really looking forward to the next iMac "pro" or whatever it'll be called. Decisions!