I have to disagree, I think a variable cap is a bad idea. If there is a problem with the cost of soulstones vs. troops, that should be addressed by the rules (like having unspent soulstones only transfer to the cache at a 2-1 ratio or something).
My concern is that the number of soulstones a model possesses is directly related to their ability to survive as the rules are presently written. I would bet on almost any master with eight soulstones against any other master with only four. In the example you give, Sonia Criid would wipe the floor with Perdita with that many extra stones.
The real problem (if you view it as a problem, I'm not sure I do) is that almost any master, given a few soulstones and decent cards, can beat any non-master pretty easily. If a master with three soulstones can easily take out a 9 or 10 soulstone model, why wouldn't you max out the number of soulstones you're allowed?
If you ask me, the masters should be tough. They are the heroes of the story. However, that said, there probably needs to be a pretty thorough re-think of all of the strategies and schemes as they don't seem well balanced. Some are really hard to accomplish (slaughter, for instance, is really difficult in a big game. A player can almost always hide a model somewhere). Others are much easier. Perhaps there should be a starting soulstone bonus or penalty depending on the strategy you draw.