SG Nerf had made aluminum replacement parts for some of their internals. From a bolt sled for a Longshot to a Plunger tube on a recon, their results looked pretty legit and led to stronger and better internals for these blasters. I started seeing those in 2009/2010.
Fast forward to '11 and now there's a site called Orange Mod Works that is developing upgrade kits for N-Strike Blasters. These kits replace key parts with custom-fabricated aluminium parts for a better seal, stronger parts, and overall better performance. They started out with the tried-and-true Nerf Recon CS-6 and have announced other N-Strike Blasters as well. They've begun to release these online at their website and reviews of their products I've seen thus far have been favorable. I've been thinking of upgrading one of my own blasters, but I'll have to wait until they've released the kit for it: the Longstrike.
Still, it's a neat site that offers products I've yet to see sold like this. I suppose the only downside is that the prices are about that of a brand new Nerf Blaster. The Alpha Trooper "Stage One" kit sells for 18.99. I think my Target has Alpha Troopers on sale for 19.99/17.99 depending on the sales. Basic modifications for AR Removal will get you a notable performance boost already, but the more I look at OMD, the more I think it's worth the investment for a great blaster.
The only trouble is that I tend to switch primary blasters pretty frequently... and I would have a hard time deciding which of my blasters to purchase this kit upgrade for!