They will have a minor drop in velocity but accuracy is not generally affected by shorter bbls. I think some Paratroopers will be more accurate than some standard length SKSs and vice-versa.
They are very handy and make the SKS into a true carbine. I don't have a Paratrooper yet but I have 9 other SKSs from 4 different nations. If you find a deal on one, buy it! They are nice and worth a little more than your average chinese SKS. I have been thinking about making my Norinco into a Paratrooper but it has a bayonet lug and I don't want to loose that. It is also all matching and original so I will find a beater to chop up if I decide to go that route. If I found a Paratrooper for a good price, I would buy that before I bought any other SKS right now.