I think adding a completely waterproof bag behind the seat is a great upgrade, offering a lot of utility.  Because the bag is completely in the slipstream behind the seat, there is no drag penalty.  In fact, there's a chance of a very slight drag reduction if the bag helps the airflow stay attached longer.

The lightweight coated nylon pack cloth is much lighter than the heavy but indestructible PVC dry bags.  At a little over 14 ounces, it's a lot lighter than the 3 pounds added by most panniers.  The dry bag has plenty of room for a pile jacket, lunch, and lots of other stuff.

So far, I used the bag to carry my new GPS over to a friends house for show and tell.  The second trip I carried a lot of Swedish ivy cuttings back to my house for transplantation.  Working bikes need storage space.

Believe it or not, the bag shown in the picture is large enough for me to carry everything I need for an overnight camping trip, and perhaps even a two day camping trip.  I pack light.  I use a small Goretex bivy sack that rolls up into a four inch diameter cylinder that's ten inches long.  I have a summer weight sleeping bag that stuffs into a six inch diameter cylinder that's also ten inches long.  My cook stove weighs 3.5 ounces, less fuel.  I'd probably stuff the sleeping bag in the waterproof trunk bag, and roll my Thermorest ground pad and bivy sack into a six inch diameter cylinder that's 24 inches long, and I'd strap that to the back of the trunk bag.

It's best to carry weight low on a bike to improve handling and stability, but I've always preferred the bulk to be behind me so it isn't creating drag.  I haven't had handling problems with high center of gravity, but I don't carry a lot of weight.

 

The lower two corners of the bag have grommets that are very useful for mounting the bag to the seat frame.  I used reusable cable ties for convenience.  It only takes about twenty seconds to remove or install the bag on the back of the BikeE's Sweet Seat.

Unfortunately, it would also be very quick and convenient for a thief.  For a more permanent mounting, use Kevlar cord from a climbing shop, and put five minute epoxy inside the knot.

 

The seat back mounting at the top is strictly low rent.  It would be great if the bag had a couple of grommets near the top, but it doesn't.  If I get some motivation, it might in the future.  For now, I just roll the top closure to seal the bag, and then snap the closure around the horizontal bar near the top of the Sweet Seat.  The bag is a little bit taller than shown in the photo, but is essentially compressed to be able to clip around the seat bar.  The bag changes sizes automatically to accommodate loads of different volume.  The snap closure is very secure, and the aft portion of the bike frame makes a good shelf to support the bottom of the bag if needed.  I'd guess 25 pounds to be the practical weight limit for the bag in this application.

 

My assistant is the inquisitive kid from next door.  He's demonstrating the simple roll top waterproof closure method.  For an adult, the bag would be a lot more vertical with no chance of stuff dropping out when closing the bag.

Rolling the top of the bag requires about the same amount of time as closing a zipper.  It's very fast, and absolutely waterproof.  Air is captured in the process, which can help cushion delicate items.  It also allows the bag to be smooth with low aerodynamic drag regardless of how little or how much is being carried.

 

In this photo, my assistant is preparing to clip the top of the bag to the upper horizontal tube on the Sweet Seat.

Note the Northwest River Systems logo turned toward the seat.  No need advertising for them.

Even though a black bag would have matched the bike's Goth color scheme, the red is starting to grow on me.  It is a lot more attention grabbing, so my chances of being seen by a driver seem a lot better.