To get meaningful boost numbers the engine really has to be under load as Manny said. The deal is that the turbo is primarily powered by the expansion of gasses across the hot side and there isn't much heat at idle or when free-revving the engine so the boost numbers don't mean much. Given that you have a boost number I'm guessing that you have a gage. If not you'll need one.

You can get a manual boost controller if you want to boost the boost (my Vibrant was bummer and is no longer offered). It just plumbs from the factory wastegate (on the side of the turbo) and the intake manifold. You can tune it on a deserted stretch of road though I did mine on a dyno as Mutt isn't anything like street legal. I wouldn't go over 15 PSI on an otherwise stock engine and you'll run out of fuel pretty quickly so even 15 may be too much. I'm sure Manny's right when he says 10ish is the stock setting.

I'd be surprised if exhaust mods would gain you much unless yours has a restriction of some sort - flattened pipe, loose muffler baffle - that sort of thing. When you get up to much higher HP levels exhaust comes into play but most cars have plenty of exhaust capability for basic mods. Lots of guys do like to make more noise, though - but you're driving a MERCEDES fer Pete's sake!

Good luck

Dan