The new one's, the classic versions have the plenum installed from the factory and the dual transfer ports as well. Those are your biggest increases besides a longer barrel. Huma pin probe and hammer spring also help.
The older Dreamlines don't have these, so if your buying used, be sure to ask, because those are $30 and $40 each. . Be sure also to make sure that a used one has the new FX superior liner in it as well, because there have been several upgrades to the Dreamline versions. It's not a good deal to save $200 on a 3 year old gun with no warranty when you could have bought a new one for the same price with a warranty and all the upgrades included. Utah Air has some deals right now.
The Huma slug/pin probe helps some. The barrel will help as well at about 10 foot pounds depending on caliber.