is 10 the limit?

[Deus ret.]

AFAIK influence of diplomats is a skill good for two things:

(1) it decreases the diplomat's chance of falling victim to assassination.

(2) as for negotiations, it only comes into effect when negotiating amounts of denarii to be paid/received. The higher the diplomat's influence, the more likely it is for the trade partner to accept a money-based offer like map-selling (thus it counts as a kind of "bargain" skill).