Hmm. Your ISP is incorrect in telling you that removing any of your routers on the local area network will have anything to do with the bandwidth performance that they provide you at the Internet modem. On the other hand, changing/removing networking equipment on your local network DOES affect bandwidth performance on that local network.
Most likely what the ISP is asking you to do is verify that you are getting their promised download/upload bandwidth at the modem. It appears that you did that and verified that you are getting the stated bandwidth.
So, we should take a look at why, when adding the AirPort Extreme, you are getting less.
First off, which AirPort Extreme model do you have? What is the make & model of the broadband modem that your ISP is providing you?
It is always a good idea to perform a complete power recycle of your networking equipment when introducing new hardware. Please check out the following AirPort User Tip for details.
(p.s. Please don't use all capital letters when you post. It appears to readers that you are shouting. Thx!)