I just installed the 2.5 inch RB exhaust on my 88 10th AE. I also put in an S5 Turbo with a ported wastegate. It took some time to get everything right but it was worth it. I found an oil leak going to the turbo. That probably explains the higher consumption of oil.
The car sounds great and drives even better. I high recommend upgrading to anyone who hasn't.
I do notice a rattle in higher RPM range in 2nd and 3rd. It goes away when I let off the throttle. I suspect a heatshield. The boost reads 380mmHg at max output (7.3 psi). Previous was around 300mmHg. I do not know the absolute limit of gauge but it appears boost creep will not be an issue.

 

I also recently installed the rb exhaust and notice the rattle. Not sure what it is either

 

I will have to investigate but in my case I had to leave off one of two heatshields for the turbo as the S5 is different. That may be the problem. Did you go with the RevII or the 2.5? Does it rattle in a similar fashion?

 

The first set of heat shields are at the firewall and go under the car.
The next "set" is for each of the mufflers which I found can rot at the bolts attaching them to the chassis.
Very easy to address,so it won't be hard to do if that is the case.

 

My weird rattle was the top mount intercooler heat shield.