Hmm... the effects of source holdings on actions is fairly limited, with the very big exception of realm spells (I neeeeeeeeeeeeeeeeeed a 3rd level source).
However, I see your point. There is already a limit on modifiers from friendly holding - the province level. If you reduce that by half, it changes things... hardness should likely be reduced accordingly.
I don't know though, is this a problem? This is supposed to be the big advantage of higher level holdings, as I understand it - the RP and GB gain is very long term. If you want to increase your income noticably, you pretty much have to go to war (unless you're a guilder... trade routes!).
Btw, I assume you all know that the only reason Jon is apparantly agreeing with me is because we're Danish (not the damn cake!). Gogo Danish alliance!