PowerTap Pro Hub

If the bike is stock, then you have a shimano drivetrain. The rear wheel is a 28 hole rim. I am not sure if it could be rebuilt with a PT. Someone with a bit more knowledge of wheel building will probably be able to help with that. 

I went through a similar decision a few months ago.  I believe that you can build a powertap into a TTR3 rim, but you'd need all new spokes and a full wheel build anyway.  Why not just buy a new rim and have a wheel built around your powertap?  then, you still have the TTR3 as an extra wheel- which you could sell, or keep as a spare.

The TTR3 is a fine wheel.  More aero than you'd think, but it's not light.  I think my rear TTR3 weighed in at 1060 or 1070g, with rim tape.  I had FairWheel bikes build me a wheel with a 38mm aero rim- the Kinlin XR380 (550g), and with Sapim X-ray spokes and alloy nipples and with my powertap SL+ (416g) came in at just 40g more.  the Kinlin is not expensive ($120?) and at 38mm deep it comes close to matching the depth of the 40mm TTR3 front wheel... for looks.

Of course- you could spend a ton on more aero, light, deeper carbon wheels to wrap around your new powertap, but it sounds like you're just looking for a good training wheel.  You can always throw a wheelbuilder disc cover on it ($100) for race day and have the best of both worlds.

Depending on your weight, riding conditions, how bulletproof you want it, and the rim you choose- you certainly may be able to go with fewer spokes than 28.  Discuss this with your wheelbuilder.  Spokes only weigh like 4g each, so adding 4 more isn't a huge difference in weight if you choose a light rim.  I presume that Felt put 28 spokes on that beast of a rim so that they can sell the bike to anyone at any weight.  It's a great, bombproof training wheel- as it should be for a stock wheel.