there is a difference when using swamps IDM but probably not enough to warrant the current premium.
when I bought my swamps IDM it was much cheaper than it is now. main thing you will notice is the throttle is more sensitive at initial press/more responsive. not sure how to explain it.
You will not notice any power gains, etc; at least from my experience.
stock IDM's can be found on ebay or in classifieds for $120 and lower. I personally wouldn't buy a HVHF IDM unless you get a killer deal on one. I think there are some stock IDM's listed in the classified section here and other forums.
best of luck