I don't have a single issue with running my pressure source from the manifold, and I know many people who do it also.
And I have a light switch turbo, in the fact it's nothing nothingn othing everything all at once. Doesn't spike one bit.
From down near the intercooler, I'd say is actually more hose then running straight across the firewall.
Also, I didn't think the waste gate allowed the air from the pressure source line to "vent" to atmosphere. Therefore, pressure along that line should always be the same no matter what.
Also, if your boost guage is T'd from before the cooler, how do you know how much boost you're running at the motor?
Like my car runs a map sensor, so if I take my boost reading before the FMIC, where the temperature is roughly going to be the same (Hence, at same pressure etc, same amount of air is there) but my FMIC is working a lot better as it's colder, suddenly my motor sees 2 PSI less then I want it too, the MAP sensor picks up on this, and will make the car run in a different load point on the MAP... Shortening the engines life... And hey, I know I'd rather replace a $2000 turbo then a $7000 - $10000 motor... Also, if you're running that high up near the top of where you should be running the turbo, that pushing one or two more PSI is going to damage it, then really, you need to rethink your logic of choice of turbo, and pressure you're running...
Also, are you saying your boost guage, you don't let it read into vacuum?