Sounds like the carbs need cleaning. The needing to rev hard to move, sputtering, all things my wife's Shadow did before a carb cleaning. I had the carbs cleaned on my 05 back in 2010 when I got it, and with a little fuel stabilizer each fall for a couple of tanks, [knock on wood] I haven't had a carb problem since.
I also recommend shimming the jets when you do so. This will remove some of the Kawasaki cold-bloodedness. My 05 VN800B doesn't need the choke on at anything over 60 degrees out, and runs like a scalded dog.
Just my $.02
2016 Vaquero (mostly stock)
2005 VN800B Bobber (Blue Collar Bobbers kits)