"Do you need to use a pad to provide extra support or cushioning to the back area? It seems pretty light for what you get, so I'm guessing that there must be either limited cushioning on the hipbelt or the back area."
There is no cushioning on the back area, just the 420 d packcloth, but it has a soft brushed texture (like Driclime) to it rather than a smooth one. The hipbelt is cushioned with 3/8 inch thick foam core. The length of that foam core increases as the size (length) of the hipbelt increases.
How many folks go camping without some sort of sleeping pad? That pad has to be carried someplace, so why not let it do double or triple duty? So, need? No. But in my opinion, yes, most folks will want some amount of added padding to cushion their back from the hard gear inside the pack.
One of the great design features of the Traveler is its flexibility. You can use it without the stays or pad as a totally frameless rucksack. You can add varying thickness of padding to both cushion your back from your load and to form a virtual frame to transfer weight to your hips. SMD states that above 35 pounds, you might want the two optional removeable aluminum stays (that adds 5 ounces total) to transform it into a true internal frame pack.
Only you can determine how much you can comfortably carry on your shoulders alone and when to add more pad or the stays. Personally, I leave the two optional aluminum stays in place all the time, but I routinely start with about 30 pounds on a 7 day trek. Your comfort level may vary.
"I tend to carry an inflatable 2.5" pad (POE) which I don't think would work very well with this sort of system. What do you use?"
I use two different pads, depending on where I'm going. Wherever possible, I too use a 2.5 inch Big Agnes inflatable pad. It fits the pad pocket nicely, although Ron has said that any inflatable will work – once it's in place, inflate it slightly and close the valve. His web site says:
"Pads Supported
3/4 Length: Closed Cell Foam
3/4 or Full Length: Inflatable Pads"
I am experimenting with putting a single layer of 3/8 inch foam between my deflated pad and the inside of the pad pocket, just to protect the pad from any hard gear in the pack bag. If it works, I plan to use the single layer as a sit pad in camp and at the door to my tent. My wife, who carries a much lighter load that her packmule hubby, uses both stays and two single pieces of 3/8 foam (4.5 oz) in her pad pocket, and carries her vulnerable inflatable pad in the packbag. If I don't have to carry that bulky bear canister, thereby giving me space in the packbag, I'll do the same.
I also use a closed cell RidgeRest pad where there's a lot of sharp objects everywhere and thus a good chance of puncturing the inflatable (like the desert sections of the PCT) or when I absolutely must cut another 18 ounces off my load. A 3/4 length 48" RR folded into quarters fits nicely. I have a full 72" RR that I have not tried, but which I suspect might also fit in there. Be aware that whatever space your pad takes up, it removes from the interior depth of the packbag, so plan accordingly. With the 3/4 pad, I can still fit my bear canister (9" diameter)inside; I doubt I could do that using the 72" RR pad.
Bottom line – ALWAYS test any pack, using your own gear and the heaviest weight you can expect to carry on your up-coming trek. Find out what arrangement works best for YOU. CONFIRM your findings with an overnight or weekend shake-down hike before you get on the long trail.