bumble - yes teething could defo. be a reason for baby being fussy. Here is the link for fussy babies have a look at\link{http://www.kellymom.com/bf/concerns/baby/fussy-while-nursing.html\this}
alfie - I don't think formula helps a baby sleep better. I don't have experience with formula, but did wonder about it helping babies sleep better, but I did some reserach on it (LLL, kellymom.com, etc) and the bf experts say it is not supported by any sceintific evidence that formula fed babies sleep longer.
I know how hard it is with lack of sleep, I suffered from it too for a very long time, but honestly it is worth persisting with bf for 1st 6 months. I became convinced that formula/solids DO NOT help a baby sleep better, I never gave them a try. People may give you anecdotal stories of how it helped their babies sleep better, but it is not supported by anything scientific.
Be reassurred that it is just a passing phase. DS is now 21 months old (still bf) and I can't even remember the dark old days of lack of sleep 
Plus you both only have a few more weeks to go before you can give solids, why waste time with formula now ? If anything excl. bf babies often react badly to formula, their tummies aren't used to it, they get stomach upsets, constipation, etc, which will only result in more discomfort to you and the baby and possibly even more sleepless nights.
Sorry, not trying to be pushy, but please weigh the postive things about excl. bf for 1st 6 months with the potential negative things of intro. solids too soon or formula, incl. the risk of allergies.
Best of luck to both of you !