Cast the rod with 30ft of line out. With typical WF lines you will be in the middle of the belly of the taper Then cast it with 25 ft of the same line out. That will give you a rough idea.

I saw a post from Bruce Richards of Scientific Anglers once where he wrote that for typical WF lines the weight of 5ft of,the belly section was about equal to a 1 weight difference.

Line weight is rating is just a measure of the mass of a line. In the above example you reduce the mass of line being cast by about 1 line weight. If the cast was done with 35ft it's roughly like going up size.

Typically if you go down a line size the rod may feel like it has a bit faster action. The action really hasn't changed. You are just casting a lighter mass of flyline so the rod does not load (bend) as much so it unloads (unbends) in less time. Also at short distances it may be even more difficult to load the rod for a cast. Some people who are casting long distances may use lighter lines to make it easier to carry long lengths of line in the air while false casting prior to a presentation cast.