Chimbu,
Agree on the Tips as a cheap speed mod as far as speed mod prices go. They still don't come cheap from what I hear.
I don't know what research you have done on them. Don't forget that there is an empty weight penalty with the tips, prob about 25 to 30 lbs. What I'm saying is you don't get to use all of the increased take off weight as payload. Also in some tip tank installations the increased weight must be as fuel in the tips, i.e. the increased weight cannot apply if the tips are empty.