Have you sent off a note to/called Dwan or Dan at Co-Motion to get their recommendations?
There are a lot of ways to fine tune the performance of your brakes and several alternative brake designs that might work. Dwan & Dan will most likely have encountered your very same question in the past and have an answer ready for you. Moreover, if you do get into parts swapping perhaps they can give you 'Bro' pricing or do an exchange.
As for switching to Shimano's V-brakes, Avid's Single Digit series of brakes are -- for all intents and purposes -- the same "type" of brake and will give you similar performance. Therefore, before heading down that road I'd explore some other options:
1. Make sure your cable routing and the Travel Agent (what I think you're referring to as a roller booster) linerar pull adapter are working smoothly and that there's not any excess friction or kinks in your cable runs.
2. Make sure you've got enough brake lever travel to give you good modulation and maximum purchase as you use the brakes. I like to have my levers set up to move a cm or so before the pads begin to hook up, but not so much that the brake lever contacts the bar before they are locked up.
3. Different brake pads (although, Avid's Rim Wranglers are pretty good); again, get Dwan or Dan's recommendations on which compound they've found works best with your Velocity Dyad rims.
4. Keep in mind, any type of linear pull brakes and forks / rear stays will struggle push against each other when you apply max braking and the forks and stays will actually begin to be pushed outward at a certain point. If fact, this is why many folks have problems with brake squeal & linear pull brakes. So, you may want to consider trying one of daVinci's "Little Stiffy's" to "boost" your brakes performance before investing in new arms. IMHO, daVinci's boosters are the most aesthetically pleasing of the bunch and there's no question that they are stiff: http://www.davincitandems.com/index.html
5. Before spending money on the same type of brake consider putting a dual pivot brake caliper (road bike brake arch) on the front wheel and -- if the rear brake bridge on your Speedster has a hole for a pivot brake -- on the rear wheel too. The current crop of Shimano dual pivot brake arches (touring model, 105, Ultegra & DuraAce) will all perform as well or better than any linear pull brake. Linear pull brakes came into vogue because they allow tourists to easily mount fenders (mud guards for the folks across the pond) and LARGE DIAMETER tires, whereas calipers are a bit more restrictive. From what I've read in your previous postings on tires, having room for 32mm tires doesn't seem to be a priority. However, if you wanted to have future adaptability for fenders, the touring model of Shimano's brake arches easily accommodates fenders & up to a 28mm tire.
6. A rear drum brake is a good idea if you expect to find yourselves touring in the Rockies, Alps, or some other mountainous terrain where you need a drag brake to scrub off speed on long and/or very steep descents where over use of rim brakes would surely superheat your rims and cause a tire to blow out. However, in most parts of Texas that I've seen it merely be a parking brake on your bike, impractical to use as a "performance brake".
7. Avid mechanical brakes as a rear primary brake are becoming a popular option on Co-Motions. Not sure if your Speedster even came with the International Standard (I.S. or I.S.O.) disc brake mount but if it did that's another slick option when combined with the aforementioned front dual pivot rim caliper.
As you can see, lots of options. But, I think you'd be well served to speak with the folks who made your tandem and spec'd the brakes that are on it first before experimenting.