What a blow-off valve does, effectively, is de-preasurizing the turbo faster.
A turbo does not inject exhaust gas into the engine (the EGR valve, if the Metris has one, which I assume it does, does that).
What a turbo does is takes the hot, fast moving, high preassure exhaust gas and runs it through a turbine (hence the name) which is sort of like a windmill. The gas, thus, turns an axle rod which is connected to what is basically the turbine in reverse (the vanes point the opposite direction, thus creating inductive force) called an "impeller". The impeller speeds up and thus pressurizes the (external induction) air, to a pressure governed by a wastegate (it opens slightly and reduces preassure if it goes above the desired parameter.
What the turbo does, effectively, is puts more air into the cylinder, which allows you to put more fuel into that cylinder and still have a "stoichiometric" mixture of air and fuel (that is, the ideal mixture to create maximum power and complete combustion). What turbos used to do, with simple fuel injection or a carburetor , single fire spark plugs, simple vane geometry and a purely mechanical waste gate, is create a smaller engine that whenever the turbo is in use (generally by mechanically chaining operation to a certain rev rage), provides the power (and fuel consumption) of a larger engine, but allows to you to get the benefits of the smaller engine when less aggressively driven.
What the combination of direct fuel injection, variable vane turbo technology, and multi-fire spark plugs (especially when combined with variable valve timing and lift), as you find in the Metris, it creates an engine that is effectively multi displacement. It can achieve, by varying the use and pressure of the turbo, and using the direct fuel injection to perfectly meter fuel, stoichiometry at naturally aspirated air pressure for the two-liter four, or it can do it with the heavy pressure and a ton of fuel- which is how the AMG version of the Metris's engine can make 380 bhp effortlessly.
So unlike the cylinder-deactiviation method of roughly doing the same thing, the Metris has the dead weight- and more importantly- reciprocating mass of a 2-liter four, but can provide the combustion to provide the horsepower and torque characteristics of a 1990s Ford 4.6 V8. Whereas trying to cut the power of the Ford V-8 to a 2.3 liter 4-cylinder by shutting down half the cylinders would still leave you with the reciprocating mass and dead weight of the 4.6 V8.
The downside is, of course, that the Metris will always have the inherent imbalance and lack of smoothness of a 2.0 liter 4-cylinder (which Mercedes attempts to partially quell using a Lanchester balancing module), whereas the Ford V8 will still have the more balanced smoothness inherent to having 8 cylinders in motion instead of four.
What a blow-off valve does is, upon the turbo no longer being called upon for use, it immediately dumps the pressurized induction (clean) air. This creates a really cool sound. It doesn't solve any "problems" involving hot exhaust gas (which will continue to go into the exhaust system for treatment, and bypassing such a thing would be extremely illegal in all 50 states). It does have the potential to increase turbo lag (if you lift off the throttle completely, and then stop on it in short order, the turbo, which would otherwise still be at least somewhat pressurized, would have to fully respool again). You will be dumping relatively cool air directly into the engine bay. (don't think it does much if anything). You might hasten wear on the turbo due to changing pressures or temperatures being magnified (not sure about this) which could theoretically be used as a reason to not honor a warranty.
I'm sure if you had a problem they could trace directly to the effects of the blow off valve, you will have some trouble getting the warranty honored. Can't imagine what they would be, but I really doubt the deleterious effects of the valve (and there are some, mostly relating to increased heat/preassure flux and strain wear on the turbo) would be drastic enough that they would show up under the warranty period.
