TurboD,
Whoa, there! One thing at a time!
Now, let's see:
1- Stock boost should be 0.85-0.95 bar, at full load, in third gear, at 4000rpm.
2- This can be adjusted, but not particularly easily. If you have the KKK K24 turbo, you can adjust it with the screw in the end of the wastegate (accessible from underneath the car). This turbo has a small metal pipe going to the wastegate. If you have the Garrett turbo, you need to adjust the length of the rod between the wastegate pressure canister and the valve itself. This turbo has a rubber hose from the intake to the wastegate canister. Sorry, I don't have more info or photos of the Garrett (yet). Pictures of the KKK turbo can be found here:
http://www.meimann.com/images/mercedes/head_gasket/
3- I don't know for sure what the HP gain would be. I do know that the engine is rated at 148hp when running as Mercedes intended it to run. It depends how "bad" yours is now. The best test, IMO, is to clock the car 0-60. At sea level, on a flat road, the spec is 10.9 seconds. If yours is 13 seconds or less it's pretty good. Slower than ~13-14 seconds means you need to fix *something* - the trick is to figure out what! Note: At higher elevations (over 2000-3000 ft), there will be a significant power loss off idle. Not much you can do about that though.
4- The max boost capability of the stock engine is whatever it will take before melting the pistons. The overboost switch on my car is stamped "1.1 bar +/- 0.15". So, Mercedes put a safety rated at 1.25 bar max. I don't know if I'd want my car running at that amount on a daily basis, though. That's asking for a blown motor ($8000 for a new BARE longblock, or $8000 for a rebuilt with injectors, IP, and turbo.)
5- I'm about to mess with mine a little, but haven't had time yet. I want to set mine around 1.0-1.1 bar max. I tested my overboost sensor by hand with a MityVac and it engages at 1.2-1.25 bar.
6- No, the factory service manual does NOT specify how to adjust boost. It is supposed to be set by the turbo manufacturer. The only procedure is how to test the boost pressure. You need to figure out how to adjust it yourself.
7- There's not much else to increase performance over the factory rating. What you want to do is make sure you are actually getting the full rated 148hp. That means having the trap oxidizer removed (factory recall) if it hasn't been already, disabling the EGR and air recirc valves, making sure your injectors are good (can be tested for ~$30-40 at a diesel shop), cleaning out the intake manifold if necessary, cleaning out the hoses, tubing, and valves between the intake manifold and ALDA (on the injection pump), making sure you don't have brakes dragging, using synthetic oil in the engine, tranny, and rear end, keeping tires inflated properly, etc, etc. One thing I have heard, but not verified, is that the "cruise control surge retrofit" can reduce power output. If your car has 3 switchover valves in the pressure/vac tubing near the washer reservoir, your car has the retrofit. If you only have 2 it doesn't. My one '87 has this but I haven't removed it yet as a test. It's designed to compensate for the air recirc valve, but if that is disabled, it's not needed.
8- If the car is slow off the line (and you are near sea level), you can either remove the injection pump (I/P) and have it calibrated (big job), or try tweaking the ALDA on top of the I/P. Search the archives for details on how to do this. This will NOT, I repeat NOT improve top end power. It will ONLY increase "off idle" power. But it makes a big difference if yours is weak off the line! Remember, this applies at sea level more than at higher elevations...
Please post again if you experiment, to let is know what happens. Good luck!