I think the placebo effect has been increased.
But on a serious note, I haven't noticed any changes in the sounds. I also wonder if changes in the sound would not require a rather big update, in terms of filesize? Well, I know nothing about how they built the sound in GT5, but I know that sound samples can be quite big, especially if they're supposed to alter in pitch and dynamics and such (like an engine sound would have to). I know that the sound databank for the notation software Sibelius 7 weighs in at 38 gb, but that is covering pretty much every instrument known to man... And the soundbank I'm using for my notation software is 140 mb, which is average for a standard MIDI set in a rather low quality. A soundbank with good quality can easily be 30 mb per instrument, which in this case would be 30 mb per type of engine. Unless they have some smart way to compress the filesizes of those things, which they probably have...
I'm also guessing that the sound bank of GT5 can't just be changed in small patches, if they needed to change something in the sound bank, they would need a patch which deletes the entire sound bank and then installs a new one. Again, guessing. I only know how these things work in notation softwares
But small changes, in like volume, reverb, doppler etc can probably be quite small patches, as it's only fiddling with the numbers in some file somewhere, and not actual changes to any sound files.
Edit: To clarify (BUT STILL GUESSING!). Sound databanks are often quite complex (computers are made to work with visual content, not audio), and alterations in the actual sound bank would often require a special software to be used. If you for instance want to swap an old and low quality V8 sound to a new and improved V8 sound, you would need to do that in a special software that can open the bank and work inside it. That's why I don't think they can change an engine sound here and there through patches, I think they would need to take the old sound bank, open it up with some software at the PD workshop and do the changes there. Then construct a patch which removes the old sound bank and installs the new one. My guess is therefor that when we see a sound bank update, it would weigh in on at least 1 gigabyte.