How to test wireless card on MacBook Pro?

My guess would be that it's not hardware related - that would likely result in a loss of connectivity, rather than slow connectivity. More likely, it's software or firmware related - it might be that the 10.5.8 issues affect more models than Apple addressed with the update I linked above, or it's a different problem entirely. There are similar posts in this forum from other models (e.g. a 2007 MBP) (but it's not all models, as I updated to 10.5.8 this morning, and my speeds are fine).

If you're near an Apple Store, I'd recommend taking it in.

First off, have you installed this?

http://support.apple.com/downloads/AirPortClient_Update_for_MacBook_and_MacBookPro

It was released yesterday and corrects a known slow wireless issue for the 17" Late 2008 MBPs (among a few other models) caused by the 10.5.8 update.

No - since the 15" Late 2008 MBP is not listed, you should not try to install it manually. Also, that issue was only present when the MBP was running on battery power. Is that the case for you, or does the same slowness occur on AC?

How are you determining that your MBP is running slow - something like speedtest.net, comparing to other machines in the house?

But, how do you know you are getting 5-11 Mbps? Do keep in mind that even you're running 802.11n wireless, which is rated at up to 300 Mbps, that's only the speed from the router to your computer. Most home internet connections are from 1.5 - 10 Mbps. Even the fastest US fiber optic home internet runs at 50 Mbps (meaning even 802.11g wireless is faster).

Try a site like speedtest.net - what values are reported?

Yes, the AHT does test the Airport Card.