Re-routing Rear Gear cable....

My bike currently has downtube, bottom bracket, chainstay mounts the rear gear cable but even with sealed cables the shifting is sloppy and slow sometimes.
My current thinkin is to get a length of outer cable and run it full length to the rear mech over the toptube and down the seatstay, i dont have any cable guides or stops to allow this but will use cable ties to hold it in place.
Are there any potential problems im likely to run into that i havent thought of? As far as i can see there shouldn't be any problems whatsoever but i'd like some opinions from those that have incase i've overlooked somethin simple.
Cheers guys and gals
My current thinkin is to get a length of outer cable and run it full length to the rear mech over the toptube and down the seatstay, i dont have any cable guides or stops to allow this but will use cable ties to hold it in place.
Are there any potential problems im likely to run into that i havent thought of? As far as i can see there shouldn't be any problems whatsoever but i'd like some opinions from those that have incase i've overlooked somethin simple.
Cheers guys and gals

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Posts
no troubles i came into, but made it a lot better shifting with full outers, just tied it neatly to the frame and looks good as intended!
After all, I am Cornish!
http://cornwallmtb.kk5.org/
Cotic Soul, The bike of Legends!
http://www.bikeradar.com/forums/viewtop ... 1#16297481
never a missed change through british winter slop.
wiping dry wax lube onto gear cables before drawing into outers improves general slickness 8)
of course theres no going back to split outers if you do but lets face it , you wouldn't want to TBH.
It looks like a trip to evans on sat for me
Merida
Austria 2012
Thats what I did to my Reign, made an excellent difference, shifting stays smooth with a capital smoo all year round!
I'm just in the process of replacing my outers with some shiney braided ones, they're a bit wider than my standard old XTR outers so the files are coming back out this eve...
Rockhopper Pro - XC and commuting
DH8 - New toy :-)