We re-cut our mainsails luff curve this week. I guess we used 4 hours for the whole process. A beachcat jib and main is probably such a small order that a sailmaker would quickly loose money on modifications.
If I was to buy a set of new custom sails I would have gone to a sailmaker who knows beachcats intimately. It's not rocket science, so with some experience the sails should be good on first try. If the sails are poor you have not got what you ordered and you and the sailmaker should be able to work out some solution if the sailmaker wants to retain his reputation.

When it comes to modifying sails, there is a lot that can be done, especially with cross-cut sails (new cloths make this attractive again). But it takes time, and a sailmakers time is money..

I would not worry too much about this, but rather have checked out local sailmakers and the experience others here have with custom sails and which sailmakers have delivered what. Give whoever you decide on mast bend measured to their spesification, boat properties, crew weight, experience and ambitions and budget and you will be OK. Just dont go to a monohull sailmaker or a sailmaker with little verifiable beachcat experience.