Looks like an interesting scoring system, and it seems that it rather nicely balances offense and pitching.
If you do want to tweak to closer match MLB conditions, you might want to look at a linear weights-based sourcing of the coefficients. A good implementation of this is Tom Tango's weighted on base average (wOBA). Basically, it's sort of a hybrid of OBP and SLG where different events are given different weights, relative to their impact on scoring in average circumstances. The formula's "official" coefficients vary slightly with the run environment, but you can use static values that are going to be quite close in any MLB season.
The ones I use for computing wOBA on my site are:
Hmmm... Now that I'm looking at it, I realize you're very close to these values, because you count 1 for hit, and then extra for other types. So including the H in each other event, your coefficients are: