You know you could always mention to the dive shop up front that you are going to purchase your gear and hope that after some research, they could come close to matching it....... Just a thought.....
You should support your LDS whenever you can. You also have to make some common sense calls, especially when it comes to $. I can see paying 10% more but not 40%. The bit about warranty service..... not usually an issue. Don't let that be the major factor in overpaying for gear. If you buy from an authorized dealer, you will get service, period. If it isn't from an auth dealer, you better save enough to buy another one, repairs aren't cheap.
If an LDS is going to give you the cold shoulder for buying gear elsewhere, they are probably not going to be ranked really high on your list anyways. At least you aren't falling for the "graduation" program alot of LDS use.
The instructor is getting paid to instruct you either way. It sounds like you are dead set on making your purchases up front. Go ahead and test the water with them on prices. At least you gave them the opportunity.