Converting to TIFF is an intensive task... so not surprised if the CPU gets used at that point. It's what the CPU is for, after all. What you don't want to see is the CPU running at 30% or more with LR4 just sitting there, waiting to be told what to do. That means you have another program running that is hogging the CPU, and causing LR4 to bog down when it needs the CPU.
The bottom line is this, I think. If you have adequate RAM, and it's not causing the bottleneck. Then that is good. If you don't have any CPU hogging programs running, and LR4 is getting bogged down, then you have to ask yourself how much is it worth to you to upgrade the system?
I don't know enough about the Minis to make any informed comments. Also, my need for a Mac Pro is different than your needs. Because this is my profession, my timing was based as much on retiring a system that had an expired warranty as anything else. The new (refurbished) Mac Pro is a far bit faster, but that's not why I got it. I got it because it has a warranty, and I could literally swap the old one out and put the new one in its place.
I actually think an iMac may be better suited for you, at this point.
That said.... I started with a Mini, with the goal of getting a Mac Pro when I could afford it. I bought all my peripherals (printer, scanner, monitor) with the notion that one day I would retire the Mini and put a Mac Pro in its place, which I did. Because I did my research (about the peripherals) there was very little transition angst. It was almost as easy a swap as this last one. I mention this, only because if you are going to go the Mini route - think ahead to what you may be replacing it with in a few years. For a while I had quite the set up hooked up to a rather modest Mini.