This has been beat to death in several threads on this forum, two or three years ago but here it is one more time......
The warranty (or extended service agreement) is not void merely due to modifications. In order to void the warranty, the failure has to be directly related to the modification.
Having said that, there are dealerships out there that will "tell you" that it's void due to one modification or another and it would then be up to you to fight that (or find another dealership).
In this case here, your factory warranty or your extended service agreement are not void if you do your own service and you should find another dealership for when you do need to take it in.
I have used the warranty and the extended service agreement on my last three bikes and the modifications have never came into question. Mostly here in Sacramento, but once in Idaho and once in Missouri.......they've just never mentioned the mods. I pay my $50 deductable and be on my way.

Others here from your area might like to know which dealership that is that's lieing to you?