I have a 850 GLT 1997 light-pressure turbo. Do you think it would be a good idea for me to adjust the wastegate? Will I see and big improvements in the turbo? Will gas mileage suffer?

Hard to top Chris' answer for thoroughness. Your ECU will detect overboost immediately - at best you'll feel a small surge off the line, then...nothing unusual. The ECU will have made an adjustment....and maybe even have turned on your CE light.
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(98 S70 T5SE misc mods, mostly lighting) (92 940GLE)

I have been helping customers reset there wastegate to get quicker boost response off the line. I use a radiator pressure tester when doing the job. I check the amount of boost stock and then bleed the system and reset the boost off the line. I have done this job many many times in addition to the ipd ecu upgrade on high pressure turbos. It does help get the cars moving a little better. I did a wastegate adjustment on a low pressure turbo about two months ago. I checked it stock and it read about 2 1/2 lbs with a gauge. I pressurized the actuator to about 5 1/2 - 6 lbs and then shortened the waste gate rod to fit. This is an accurate way of adjusting the wastegate. Once I was done the car did have a noticable difference in power from the lower end and through the rpm's. After about ten minutes of driving and the ecu had time to go through it's code cycles, it promted a check engine light. When I read the fault codes it was the boost level. The car's enging management control system has a pre set enging boost level. " BOOST LIMITER " it is pre set to a certain amount, when the car over boost or is bouncing off the boos limiter it will show a check engine light. My customer brought the car back to me the next day and I took it down another half turn and checked the pressure again. It read about 5lbs. After resetting the code it still popped a check engine light again. I kept going in the sequence until the car was almost back down to normal. It was reading about appr. 4lbs and the engine light stayed off. The customer eventually bought one of our ecu upgrades. When doing the upgrade your boost limiter it raised so it will not recognize the higher levels and prompt the check engine light. You probably would not recognize a huge impact on gas mileage when raising your boost levels with the small amount you can only do on a light pressure turbo. The only time my customers have had changed in there milage is due to them stepping on it all the time in town to feel the power increase. I do not recommend resetting the boost levels on a light pressure turbo. It is just my experience,,, I am anxious to see what other people have come across. Have a good day.

Chris, have you ever installed a boost controller on a high pressure turbo?
I have a '96 850 Turbo and recently purchased a Dawes Device manual boost controller but have gotten cold feet and so far have decided not to install it.
I would like to know if you are familiar with the DD and if so, have you installed one without installing an upgraded ICU?
I am considering purchasing the package from IPD which contains the TME ECU, sport exhaust, K & N performance filter and engine stabilizer bushing.
I am unsure whether to install the just the DD and see what changes occur or go ahead and purchase the performance upgrade package from IPD. If I purchase the upgrades, would you recommend that I also install the boost controller?
I would appreciate your thoughts based on your experience. Thanks, Fred.

i built my own manual boost controller and installed it myself. love it. only boosting between 13-14 psi. makes a huge difference. if i turn it up one more click, makes about 15-16 psi and has the tires spinning the minte turbo hits boost till it hits 3rd gear. so i had to turn it back down. do whatever your heart desires. if you have the money, do it right, ecu, intake, exhaust. if money is an issue, go boost controller.

i have a 2.5" exhaust and intake made by me. but cannot afford ecu, so i went boost controller.
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94 854 turbo. (193 hp, 230.1 ft/lbs) numbers are higher, but till i go back to the dyno, i am unsure.