And what converters if not Cirrus Logic or AKM, Texas Instruments?
Specifications of CS4272 are comparable to ak4620 and both are top of theirs lines of CODECs (i.e. not as good as theirs top ADCs and DACs).
TI also has similar CODECs. OP-Amps can be better, as input buffer, that`s true and bad input buffer can ruin good converters (for example E-Mu USB interfaces, good converters, bad preamps). But converters can be the same level, or even lower. IMHO.
I just don`t very trust such words as "The converters are of the highest quality offering low-noise, high linearity and uncolored performance.", it can be true for CS4272 alone as well.
http://www.fractalaudio.com.au/about.php
"Studio Quality Components
Top-of-the-line, 24-bit Cirrus Logic© converters assure signal purity. Your Axe-Fx's proprietary analog front end features no electrolytic capacitors in the signal path, only high-quality film types. The output signal path likewise has no electrolytic capacitors. High-performance Analog Devices© op-amps are used in all signal paths. All this adds up to an extremely low noise floor (better than -105 dB). Your analogue amp and stomp box collection was never this quiet."
So, to be honest, it seems that converters is not top-of-the-line of Cirrus, because there is CS5381 (ADC) and CS4398 (DAC) with 120 db and CODECs (mentioned CS4272).