My view, for what it's worth, is that there appears to be no standard for clubs. i.e 1 companies 6 iron can be the same as anothers 7 or 5 in terms of length and loft. So measuring them against each other isn't allways accurate. Wilson done this recently with their Di7's. They were basically a club longer in numbers. 6 iron more like a 5 etc. So when you hit them you would think you'd suddenly gained a lot of distance. (for some reason this still really annoys me about Wilson and has put me off them for life).
As long as you are fairly happy with the distances I feel accuracy and consistency are more important than sheer length.
Its the newness of the club to a person, the fresh feel of new clubs that means you can get a few extra yards sometimes. I don't think I've experienced more than an extra 5 or 10 yards with a club. I think the last time I experienced a decent improvement with a club over a previous one is when I started using the Yonex C280 irons. I think my Pings are give or take only a few yards at the very most different, in both directions accross the set, but then you must take in differences in loft for irons of supposed same value.
In reality I doubt there would really be much difference between two sets of quality clubs where the irons are rated of similar or same lofts. Getting the right match of Iron & shaft for your swing might in a specific club make the difference of a few yards.
There are several factors that can make a difference in the performance of new clubs against old. It could be the shafts, it could be the loft, it could be the design of the cubhead and its weight dispersion. Taking all that into account along with the human factor "I've just paid Â£xxx for these so they must be better" and there may be a differnce. It may be slight or if its a new (modern ) model against an old model there could be a half to a whole club difference in distance. Whatever there is or isn't, the main thing is that you have to get out and play with the new clubs and get the feeling of them to get the best out of them. As golfers we are a marketing mans dream , we are always looking for that wee bit extra the new clubs can give us.