The obvious advantage is lighter weight
As far as sound goes it will absorb sound a little better (slightly) but not really that noticeable with standard exhaust sizing, but at what payoff ? What length are you talking about aprox? You will find that aluminium dissipates heat approx. 25x better than stainless steel, sure it is lighter but what are you trying to achieve, as you recently soundproofed your car/doors from memory, taking into account the work you have done it may cause you more problems.
Yes, it is lighter weight and it does dissipates heat very well and cools down faster then most other metals. But putting this into perspective with you recent modifications in regards heat and sound deadening done on your CJ it will dissipate the heat into the surrounding areas far more efficiently then SS/Steel, in other words it will heat up its surroundings ie your floor and or exhaust tunnel. As far as the melting point goes, aluminium begins to break up and melt around 660C , steel around 1370C and Stainless steel is around 1400-1450C depending on structure (the average car temp exhaust while driving is truly variable depending on many factors) but remember that the temp that aluminium will start to anneal (soften) is around 400-480C, dependant of course upon thickness etc.
Aluminium can withstand heat but not excessively, and over time it will start to corrode and degrade when heat is added. The aluminium also is more brittle and does not like bending or flexing well.
Not sure exact size of Mitsubishi S/steel factory exhaust pipe, guessing maybe 321ss, maybe could use 304?, but aluminium must be thicker, and taking into account the above factors depending on the length it may need an extra support in middle as when it heats up it can bend if you hit a decent size bump or pothole, i have seen this happen before when the pipe was quite hot and wasn't supported by an extra support.
Another thing is how are you doing this, are you buying a prefab unit, if you are thinking about cut/shut make sure the welder is certified to weld aluminium otherwise you will have more problems.
Lots of people run aluminium pipes without any problems for years, im just saying in your case don't overthink it too much with what mods you have done already to cut down noise/heat.
(disclaimer) all the above sizes/ temp etc quoted are all aprox as too many variables come into play in regards location/thickness etc)
