I have not tried SumoSprings, but I have used Timbren products on my Silverado 3500HD. Night and day driving experience when towing my 18000lb fifth wheel.
I realize that's not the same as the Transit 350HD, but the effect is the same. Under load, those Timbren "springs" softened the road, leveled my truck and trailer a bit more, and gave me a confident feel when driving.
When I shopped for a suspension support upgrade that was cheaper than air springs, I looked at Sumos. They just don't look durable......but that just my opinion.
2017 Gemini 23TR