The street glide is a lowered version of the electraglide so you will have about 2'' less spring travel. Having said that I have many friends with street glides that travel long distances 2 up and have no problems.
Like Steve said, it's a physical limitation due to less travel available. My wife's not very big but if she's aboard, I bottom out quite frequently going over abrupt bumps.
I've been mulling over the Progressive shock upgrade for years now but just can't justify the $400+ for the few times a year she rides with me and it becomes an issue.
Check these out... Progressive Suspension 412 Series Heavy Duty 12in. Shocks - Chrome 412-4234C:Amazon has them for $225 shipped to the house. I have several buddies running these on their SG and are very happy. They tell me Night and Day difference in the ride over stock.When I ride with them,their headlight is no longer flopping up and down and the rear doesn't bunny hop over every little bump You have to go with the Heavy Duty Spring the standard spring is too soft for a Touring bike.
Adjusting the AIR over oil has a lot to do with the final ride. But I ride an 09 King and that is a lot different than the Lowered bike...
Might think of buying take-offs off a King/glide type and have more stroke on the shocks... then raise the front IF you can't stand running Down hill...LOL
I ended up trading in my SG for an EGC. The ride/comfort on the SG was close to terrible!
Yes, I could have bought new, longer (13") shocks, but I guess I wanted something new and the price for a left over was hard to argue, so I happily got rid of the SG.
if you never owned a n electraglide or road king and bought a street glide it would probably be ok I went from a electraglide to a street glide and was terribly disappointed and didn't keep it very long road like a lumber wagon in comparison I now own a road king and it rides very well also