If the tech told you that a complete stage 1 isn't necessary, you might have found yourself a good trustworthy tech.
Aside from that, here's the info I think you're getting at:
What you have now, the slip ons, will make the bike sound better, and that is about it.
A stage 1 (pipes, air cleaner, fuel mgmt) does a couple things:
-allows more air to flow in and out of the engine, which provides more horsepower and torque
-provides the correct air/fuel ratio to the engine, making it run cooler than stock, and more efficiently.
You will not damage your bike by doing a stage 1. Actually, you will help it out, since stock bikes have a lean fuel setting from the factory (which makes it run hotter), and stage 1 corrects it.
If the bike runs nicely now, then you don't HAVE to do the stage 1. However, if you want the bike to run more efficiently and want more power for low cost, stage 1 is your ticket.