[Leica] Wanted (maybe), Nikon F-mount teleconverter

[pklein at threshinc.com (Peter Klein)]

I have an old Soligor 300mm f/5.5 which is actually very good at f/8 and 
f/11.  It has the old "universal" T-mount with Nikon mount adapter. 
Here's a 100% crop of a cell tower about 1/3 mile away. (E-30, 1/1000 
sec, handheld with image stabilization).
<http://gallery.leica-users.org/v/pklein/album170/P3264119+1.jpg.html>

On 4/3 or micro 4/3, it gives me an effective 600mm.  I was thinking 
that if I could get a teleconverter, I could get a little more reach. I 
occasionally shoot the moon, or something distant on a tripod, but not 
that often.  So this is just something to play with now and then, maybe 
train on a bird feeder.  But not something I want to sink a lot of money 
and effort into.

Vivitar 1.4x converters in manual focus Nikon F mount are $10 on E-pay.  
2.x converters are a little more.  Genuine Nikon are $60-$100-ish.  No 
OM mount adapter exists for this lens, or I wouldn't be asking this 
question (I already have a 2x Vivitar converter in OM mount).

So, would getting a teleconverter for the Soligor would be worth it, 
both in terms of price and image degradation?

--Peter