Do modifications to a sport bike increase or decrease its value?

It's far more common in America to modify your bike to make it personal to you. Some modifications are sensible upgrades to what may be a fairly basic bike. Modifications to suspension parts and exhausts are usually a good idea. Most manufacturers fit fairly basic parts. However in the bike you described I think this sounds like a bit of a lash up, the changes will have a massive effect on the handling and stability of the bike, it may well be better but unless you ride a standard bike you will not appreciate the changes. These items would generally be fine: "Yoshimura exhaust. HID headlights. Fender eliminator. Puig dark smoke front screen. Swingarm spools"   The other point to note is that adding parts should not really increase value of the bike, but might it make it more saleable. On my bikes I have usually added a different exhaust, suspension (Ohlins is a great make) and screens, crash bungs etc. However I usually remove these and replace the standard parts before selling the bike. The parts can then be sold on forums or ebay and will make more money than if they are left on the bike. Join an owners forum for makes of bikes you are interested in and ask lots of questions. These can also be a good place to find a bike for sale as the owners are more knowledgable and will have looked after their machines better.

Some quotes I found helpful from the Used Motorcycle Evaluation Guide by Adam Glass (http://www.clarity.net/adam/buying-bike.html) Heavily-modified bikes should probably be avoided Racing puts tremendous stress on machinery. You may or may not want to buy a bike that's been raced (the price ought to be way lower than it would be otherwise), but you should definitely try to find out if it has or hasn't been raced, so you can adjust the price accordingly if need be. Modifications: generally, you'd be very wise to stay away from heavily-modified bikes. Even when done by a competent professional, high-compression pistons, overbores, high-performance cams, porting, etc. all lead to reduced engine longevity (or increased maintenance, or both) in the name of increased performance. When done by amateurs, these mods are instant engine killers. Make sure you ask the seller what modifications were made to the bike.

I have noticed that generally, modified bikes have a lower value than those that are not in terms of resale value.  It is my opinion that despite the addition of expensive desirables, non OEM accessories are more likely than their OEM counterparts to be installed by someone or some shop other than the dealership.  Reliability and longevity becomes a concern at that point.

People who modify tend to be more into bikes.  Read into that what you will. As for value, surely it depends on how important those modifications are to you personally.  You might have no use for a loud can, or a HID conversion.

As a general rule of thumb you never get back the money you spent on performance parts. I am into BMW GS Bikes. A wilbers suspension is a fine thing which costs about 1,000 dollars. But a bike with Wilbers suspension will never be paid for a premium of 1,000 dollars. If I exchange parts on my bike I always store away the original parts. When trIng to sell the bike, I swap back the original parts and try to sell the performance parts separately.