I have a Widow on the way and will be considering the same thing. From my research, the "spring" rests seem to be the way to go, if you have a plunger hole.
They sell them at martinarchery.com for about $6, which is not bad for an experiment.
I also have a T300 that I have yet to mount on a riser, and it's about to become an experiment on a home-made ILF riser.
In the reading that I have done, I have yet to hear of "worse" performance from an elevated rest over shooting from the shelf, as long as the setup is tuned and the arrows are spined properly.
One interesting note: while tuning my Border, I put a piece of masking tape on the riser so I could remove the rest material without screwing up the finish, while I was shimming it in/out to tune.
The stupid thing stuck to the tape so well I left it on that way for a year, thru rain, sleet, and snow....I just trimmed the tape so the feathers wouldn't lift a corner of it...
Take Care,
Marc