[A4] 16806 cat issues?

[aaron...]

Greetings....

The short version of this story is....
I had this car for just over a year now...
1.8t KO4 mtm, 120k on it...
all this work was done about 15k ago...refurbished head, spark plugs, 
n75F, Saab diverter valve, new PRIMARY O2, new coolant temp sensor, new 
t-stat, new MAF

I have never seen above 24.5mpg, even on road trips.....during a dyno 
run it pulled a mere 170hp, and the a/f ratio dipped into the 11's...(i 
have the plot somewhere on line)....I have tested for vacuum  leaks time 
and time again....Early last week i noticed a big lack in power, and the 
max boost has dropped from 18psi down to 12psi....then yesterday it 
pulled a CEL.  "16806 Main catalyst bank 1 below efficiency threshold". 

I did some looking on Audi World and Vortex....and as usual there are a 
plethora of choices.....I am leaning towards a partially plugged cat, 
considering the age of the car, and considering how rich it runs....does 
anyone want to offer up any suggestions before i plunk down $300 on a 
new cat?  

Speaking of which I am thinking about TT's cat...it is $300 w/ all the 
hardware....it has a rating of 400cel's and the oem one is roughly 650.  
I am also debating about doing the downpipe as well for another 
$225...seeings i am in there type deal...any thoughts?

TIA...

more info on the car:
http://www.parts4vws.com/registry/registry_detail.asp?MemberCarID=11489