"hog" is a relative term. A software-based audio processor that uses 10% of the cycles of a 100MHz CPU may only use 5% of a 200MHz CPU. Back in the day, CPU power wasn't that great compared to what needed to be done. Now, we've got plenty of cycles to spare even when playing intense games. AC'97 with say, 2-channel output, really hasn't changed much. 2-channel audio only takes a certain amount of processing, and all things being equal, the efficiency of an AC'97 audio processor will become better over development time, so fewer cycles will be needed for each generation of a particular company's AC'97 solution. At the same time, the processor doing the work will have gotten hundreds of MHz faster, if not more than a GHz faster. The gap between needed cycles and available cycles grows wider and wider.
AC'97 has however changed to incorporate things like 4 channel audio, 6 channels, digital output, et cetera. That takes more cycles than standard 2-channel audio. Audio quality of AC'97 isn't nearly as good as a good hardware processor, but it is fully adequate for the average user; however when you move up to multiple channels of output, and various audio effects, the AC'97 processor will start to use more of the host CPU's cycles, and start to sound lower quality compared to a hardware processor.
Even in fast games, the CPU probably has a lot of unused cycles, so if an AC'97 audio processor only needs 1 or 2%, the hit to gaming performance or other apps isn't going to be very noticeable. I don't know if Winstone 99 uses audio output, but unless the system is actually outputting sound, the CPU isn't really doing any processing; the CPU cycles only get used when audio is being played, for the most part, so running a soundless benchmark shouldn't be affected at all.
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