How much more tire if moving springs but NOT mini tubbing?

If you're talking about a 3" spring relocation, there's no point in doing it without a mini-tub. The 1/2" spring offset from Dr. Diff gets you all the room you can use with the stock wheel tubs. Once you've installed that the tubs are the limiting factor. The offset hangers and shackles just bolt in, moving the spring perches is the only welding for that one, much easier than the 3" relocation.

This a duster we're talking about? 275's will fit with the stock spring locations. If you do the 1/2" offset and are willing to trim or roll the quarter lips back another 1/2" you can run 295's. That's what I run on my Duster. 1/2" spring offset, ~1/2" trimmed off the quarter lips, and 295/35/18's on 18x10's. Thats the biggest that will fit inside the wheel tubs, only way to go bigger would be to hang thr tires out of the quarters and jack up the ride height, or do the full 3" relocation and mini-tub.