[c-nsp] XFP, SFP+, ???

[Jeff Bacon]

So, I am looking at doing DWDM for some mid-haul passive-splitter links
around NYC metro - working with a carrier that's buying dark and
offering to hand me lambdas off passive splitters. 

I'm going to need to punch enough power to go 20-40km + losses in the
passive splitters. 

Do I need to look at SXI + SIP-400 + SPA to go XFP? 
Am I better off with a 6704 to get XENPAK even tho it's not the best
board in the known universe? 
Or do I go with X2 tuned optics? 

Effectively, what's the cheapest practical option, assuming I need no
more than 4 ports per 6500 and don't have > 1 slot available? (I have
physical space constraints, I mostly work with 03E and 04E chassis)

Thanks,
-bacon