I agree with Ted, it definitely depends on alloy. Most heat treated alloys like 6xxx loose upwards of 50-60% of their strength in the HAZ. Post weld heat treat help immensly but is very hard to implement on any scale beyond small parts and would require filler metals that can sustain that treatment.
Check out alcotecs website, each of their GMAW wires (4043 and 5356 being most common) have typical as welded strengths in a table for different alloys that they can/should be used on. This is also dependant on your skill as a welder, weld prep, and hell, even the humidity. Alcotec has some excellent reading material on the subject of aluminum. You can also check out weldreality.com, Ed's aluminum mig section is a must read.
David Todtman wrote: