Hey Mike..

Congratulations on the Fat Boy.. they are a blast.
Fatter tires do add comfort [and it might be all the comfort you need].. but they don't make up for suspension, or at least in rougher stuff. Right now I have 3 Fat bikes.. and 2 plus bikes. Fat bikes are bikes that have at least a 3.8 inch tire.. [which generally have rims that are 65mm -100mm]. Plus bikes have 3" tires on narrower rims.
My Krampus is 29er plus, and it is fully rigid. It is a blast.. I like the challenge of riding it without suspension.. but in rough east coast rooty and rocky stuff, it can be a handful. The Fuze is a 27.5 plus bike.. I don't have a lot of time on yet.. but is equally fun. I can go MUCH faster on it in the rough ruff.
I did purchase a Bluto suspension fork for my Ice Cream Truck.. a 5" tired fat bike. Again... I can go much faster with it and with more control. A 5" tire goes cushion things.. but it also gets wicked bouncy hitting the big stuff.. There is no rebound damping to a big tire filled with air... you start bouncing down the trail like a pinball.
I did purchase the 4.6 Ground control tires to try on the ICT... and love them. They roll a quite a bit better than the Lou and Bud tires it came stock with. I love the Lou and Bud when you need them... [maximum traction and float] but much of the time, I don't need that much.
I did also purchase another set of wheels so I could run 29 plus on it also. [if I had known I was getting the Fuze.. I might not have done that..but it did add to my education]
There are more efficient tires out there.. although I can't say I have ridden any of the new ones. The original Fat bike tires.. Endomorph and Big Fat Larry... had minimal tread.. and were faster, but I am not sure that either is still made. They do have more road worthy tires.. something that would be faster / more efficient, than your ground controls. Your Ground Controls would wear out fairly quickly on a lot of pavement. [meaning the knobs would wear down] which I guess would only be a problem if you were riding where you needed that traction.
You don't need to stay with the same width tire as what your bike came with... and tires do vary.. the Lou and Bud for example are listed as a 4.8... just .2 inches wider than your ground controls.. but there is a big difference in them.
I am not sure how those 3.5's would fit your bike. The issue being the rim width [you have ... if I remember correctly 82mm rims] They may work great.. I just don't have any experience with them.
Enjoy... if you have any other questions.. feel free...

I just looked at the rim spec.. they list inside width now vs outside.. but they are listed as an 86mm [so maybe they were a 92 outside] I have 65mm, 82, and 100's