I also put a Jimbo shield on my 2003 TW. Everyone should have one and I agree it makes it makes the bike look much better.
I achieved a 25MPH speed increase with mine (or, at least that is what I estimate, I was enjoying the new shield so much that I never actually had time to look down at the speedometer). However, since you got the dark grey shield you should not expect that much increase. You see I got the Crystal Clear Jimbo shield. When you have top speeds in the range of what a TW can achieve the speed of light becomes one of the limiting parameters that you must consider. My Crystal Clear shield allows the light photons to pass through unimpeded. However, your grey shield will cause excessive drag against the photons and this will attenuate any speed increase that you would gain from the advanced aerodynamics of your new shield.
So, enjoy your awesome new shield and the reduced wind buffeting and improved aesthetics that it provides but you will likely only enjoy minimal speed increases until you switch to the Crystal Clear version.
Old White Truck
And don't forget to add cow horn to supplement the Jimbo Shield!
Alex, sounds like you're having fun with the TW.