Limiting stat points is probably the most retarded idea I heard for a Holy Grail War scenario.
Stats themselves only play a minor role in FSN and Servants regularly handle others with higher parameters.
It's more important to determine the said hero's style if anything.
The actual battles are determined by special skills, noble phantasms and other such abilities interacting with each other.

You worry about Munchkins that max out all the parameters?
Don't be.
You should only use stats that make sense. Nothing more but nothing less.
For example 15 points is actually way too much for any real Assassin class Servant to have. 
Don't add extra points just because it may give you a minor advantage. This is roleplaying. Not a fuckin' videogame.
On the other hand I can barely count any decent Knightly class Servant which you can recreate by only 15 points worth of parameters.
Hell, you can't even make a so-called "average Servant" as it was described by Nasu. That'd need about 18 points.
So yeah, excuse me but I'm going to royally shit on this system.
Same with skills.
Instead of limiting by points it'd be better to comprehend what a character with all of their skills and Noble Phantasms can do and then evaluate everything based on this.
If the mix of all the various abilities are too broken, ask the player to revisit their Servant.
If it isn't, just roll with it.
If certain abilities and skills don't make sense given the Servant's history, then you're back to the drawing board again.
Really, use your brains people.