To answer your question, I don't think swapping tires will have much impact, at least not in my experience (I currently have R4). Airing down tire pressure in the rear will have more impact. That, and getting a new seat with better springs (I hate the stock seat on GC's). Pretty much what everyone else is saying.
I personally mow about 6-7 acres with MANY trees, hills, ditchlines, pond, etc. Used to mow with a Cub, did a great job, but slow and uncomfortable over time,would have to split mowing over a couple days. Went from that to a front mount JD diesel with 72"deck, cut mowing time by a bunch but also uncomfortable. 5 years ago I bought a Ferris zturn with 61" deck and full suspension, and by far the best machine I have ever mowed with. Not only cut mowing time in half, with the suspension it is by far the best ride. 5 years later i have had to only do regular maintenance and replaced 2 belts. Expensive, but worth it to me. I use the GC for everything else.