Slitherine

Reading through the rules for the first time. It seems as if leaders would be less effective in 28mm than in 15mm. That is, a leader's command radius will cover more 15mm bases than 28mm bases. Does this make an actual difference in the play of the game?

Playing with 25's is a very different game, though some people use a 40mm MU for 25mm scale. You need a lot of room then though.

I play with 25mm scale using 40mm MU on a standard 6' x 4' table (650 pts max) and it works quite right. I played once using 1" MU and, as you point it out, command and control is quite less effective. I'm actually considering buying a table tennis table (roughly 9' x 5') to play bigger games.

I'm gonna need a bigger gaming mat

When we play with 25s at my friend's home, we use a huge table and 1MU = 40mm. Basically, we scale up everything by 50%.

We found 30mm a good compromise MU for 28mm figures.
You don't need a huge table, but inches seem too small.
Also has the merit that the 2MU restricted area becomes exactly one base width, so is easy to see.