You can put headers on and get +30HP which doesn't sound like much but it's a good bit. They run IIRC $700 for the Kooks header, there is a thread here on it. You can definitely get a tune on your I6 and it will improve more than just your HP...you can decide what shift hardness you want, remove the governor which means your truck won't shut off automatically at the 105MPH mark I believe it is, and of course you can remove the torque management (see video below for an example of what I'm talking about). The possibilities are endless really. Sure you won't get a souped up truck like on fast and the furious, but it will indeed go faster, handle better etc..
I would just do as much reading and learning and get to know your Envoy before you call it quits. I mean hell the safety of these trucks compared to the yukon in my opinion are amazing, to me they look better. But you also have to remember that the Envoy is not a sport vehicle it is a luxury vehicle, you could easily get 320HP out of your Envoy, you just have to know what your doing, and have money to do it.
And yes, there are probably a lot more options with the V8....but if you want to go fast you have one of 2 options...get an SS or get a sport car. These are all things that should have been considered before buying your truck. These trucks aren't made to go balls out right from the factory, and neither is a V8. Either way, your looking at spending money, lots of it, even if you do get the yukon, not only are you out 7k but you'll be out much more money to make it go fast.
Trailblazer LL8 4.2 liter electronic throttle demonstration - YouTube