Good handlebars are generally made from specially-processed, high-strength alloy. The processing includes the tube drawing and bending processes, plus any post-bending heat-treatment.

Any kind of weld completely interrupts the molecular structure that resulted from the processing, not to mention the alloy composition itself, which would be stratified into zones in and about the weld with varying chemistry and grain structure.

A sleeve might seem to reduce the stress in the area of the weld, but are you taking torsional stress into consideration?

Welding a handlebar made from high-strength alloy withut knowing the exact alloy/chemistry and processing can't be a good idea since there is too much possible weakening of the metal.

For some reason, wide road handlebars were something of a rarity on production bikes in the old days. Some high-end handlebar brands offered wider ones, and builders of high-end bikes sometimes spec'd these bars with widths somewhat in proportion to frame size.