Not the neatest solution, but if one of your switches is POE, you could put an POE powerable mini 5 port switch (i think netgear do some.. probly others too) in an IP rated enclosure like this [Link: bluehelix.co.uk] half way allong the existing cable run outdoors.

Then you would essentially be halving the distance as you would now have a booster in the middle.

Estimated material cost : <$100 for switch and enclosure.