road bikes are much cooler than hybrids however an average speed of 18 mph solo is pretty decent (unless you have absolutely no hills, then it sux), you wont see much increase.
I would say take that 1700 and buy yourself a bare frame off ebay then piecemeal the components together, thats what I did, and got a $5000 bike for around $2600 total. sure some components are slightly used but still look and perform like new. I always find it more rewarding to build myself rather than buy off the shelf, plus I taught myself all the mechanical aspects of my ride.
as far as how many miles to make it worth it.... this is an item you may have for 30 years or more. If you have one enjoyable ride a month, every month, untill you retire...well worth doing it right.
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2010 Kestrel RT900SL, 800k carbon, chorus/record, speedplay, zonda
2000 litespeed Unicoi Ti, XTR,XT, Campy crank, time atac, carbon forks