Fairly stock 318/904 w new gears

Yes there will be a huge difference in how it drives. I know you said you don't care about highway but just for reference and while making a few assumptions, right now to go 70mph with the 2.45 you might be spinning about 2000-2200 rpm. With the 3.91 you might be spinning closer to about 3400-3600 rpm. It might not be as enjoyable at those highway speeds but you will get to that speed a hell of a lot quicker