but if you'r a geek you have to take into account the heat effected zone on either side and any undercut etc
On this part, as an FYI, the strength of the HAZ will be at least of what the weld consumable is rated for (when certifying welds consumable, whatever, this is included in the testing), or what the base material is rated, whichever is lesser. Whenver testing welds (tensile, impact, etc), all zones of the welds, the weld metal, haz, and base material, is all tested to meet at least the lowest strength requirements of the welded joint.
Situations like this have always made me question as to how strong a tube splice, welded without a sleeve and using a true full penetration weld, is when compared to a non spliced tube. Now if there is going to be a failure, it is likely going to occur at the joint due to stress risers and the fact that the weld is a stiffer section of the tube, but would it have failed anyway without the splice (whether its impact or bending load)? Because I compare this situation to that of pressure piping, or in the structural steel world. Sleeves like this are rarely/never used in the pressure piping nor in the structural would, only times you will see it when you backing for a full penetration joint for whatever reason.
Either way this is great info, Clay you need to post more of this type of thing up on here.