While I agree that could probably have been designed ,better the point is that design is sufficient for the intended use.
What you're implying is tantamount to implying the Colt SAA is a poor design because it won't handle the full potential of the 45 Colt cartridge as in Ruger level loads. What I, and others are implying is using anything beyond the limits of what it was actually designed to do, whether we think it a good design or not, and it fails is not the fault of the design.
If you want it to do more difficult tasks with your 450 then, perhaps, making a better designed modification may be the solution? That's what I would do were I so inclined. Since my 450s were doing what Lyman made them to do I went the simpler route for more difficult bullet sizing and obtained push through sizers to use in my reloading press which is designed and made to handle the job.
But back to the OPs question on the Lyman GC seater, I find it to work very well seating GCs on bullets that require GC seating. On many cast bullets the GC fits onto the base of the shank squarely and bottoms out. Thus, those bullets can be sized, GC'd and lubed in one pass through the 450. Those GCs that do not fit the shank and/or bottom out I use the GC seater to seat the GC before sizing and lubing.