IRs do indeed make up a lot of the character of the tone of a guitar amp because of the cab, but note that if you are only looking at reproducing the sound of the cab and NOT the room, IRs only need to be about 20ms in length. Anything more than that and you are also getting early reflections and characteristics of the room. However, the latter is sometimes what makes FRFR sound like an "amp in the room" which is what we are used to hearing.
The Fractal Ultra res IRs are essentially longer tail IRs which capture this room effect. But make no mistake, it does not mean it is a more accurate IR when it comes to the cab. As has been detailed in the Fractal forum, processing IRs involves convolution, and longer IRs need more power. In Ultra res, they low pass filter after 2.5kHz and decimate (downsample) so that processing is much easier for the CPU. See here for more details: http://www.thegearpage.net/board/showthread.php?t=1389699
My opinion is this is a lot of work and overkill to really just produce early reflections and reverb. It has nothing to do with what the cab contributes, which is the first 20ms. Convolution reverb has been out for a while, and though it seemed like the holy grail in providing a large number of real spaces to use, I think most recording/mix sessions have continued to use algorithmic reverbs (Lexicon, TC, etc.) and they sound just fine.
