It would be great for high rank players who have grown a liking to a certain weapon, but can't normally use it due to it's ineffectiveness against high level characters.
Maybe not in New Vegas, but something I would like to see in future Fallout games, along with the normal upgrades present in New Vegas.
For example, take the Service Rifle. By using the upgrade package, the handle would be replaced with a scope, the gun would get an extended mag (up to 30), it would gain auto fire ability, the wooden parts would be replaced by synthetic parts, and the gun would look almost new. In terms of damage, it would be multiplied anywhere from 1.2 to 1.8x, whatever seems fit.
However, the upgrade package would be pretty expensive, because of all the features added, depending on the weapon. Here's some estimated prices:
10mm Pistol: About 3,000 caps
Service Rifle: About 6,000 caps
12.5mm Submachine Gun: About 6,500 caps
Machine Gun: About 8,500 caps
Power Fist: About 10,000 caps
Rocket Launcher: About 13,500 caps
Also would be some attire upgrades. Here are some estimated prices:
Leather Armor: About 5,500 caps
Assassin Suit (Dead Money): About 9,000 caps
NCR Ranger Armor: About 11,500 caps
NCR Power Armor: About 17,500 caps
Brotherhood of Steel Power Armor: About 25,000 caps
Enclave Remnants Power Armor: About 32,000 caps
Enclave Remnants Tesla Power Armor: About 35,000 caps
What do you guys think?