Is the 5.5u buildt to take the extra loads a jib adds?
I dont know this boat, but it sounds like you have two forestays, one to each hull? If so, be warned that putting on a single forestay and a bridle strop really loads up the hulls. Unless the hulls are buildt to take this load, they will probably fail in compression.
Also, just adding a wire for the jib sheeting between hulls can rip the hulls apart unless they are buildt for it.

Very gloomy warnings, but something you really should be sure of before proceeding.

If you decide to add a jib, you should also know that your mainsail will probably be backwinded, as I guess it's much deeper in the low part than a sloop main. So you will not add as much power as you might be thinking.

If you have an old main, getting a new one might give you more power than adding a jib in your current configuration. Especially if you go for something that's not class legal (I guess you dont race one design class). Check out the "Agent Orange" thread on the F-16 forum for some inspiration.

If I had your boat, and wanted more power, I would have gotten a new large-top mainsail cut for the uni rig, and added a spi for some wild downwind sailing (the spi also loads up your hulls, but unless there was some prior negative experiences with these boats and spi's, I would have tried it).

Do you know how old your current main is, what cloth it is and how much it has been used?