Are you sure you don't mean Sylvania 6550? Can't say I've ever heard of a Siemens 6550, and I've tried pretty much every NOS KT88/6550 variant out there over time. And some google searches yield zero hits.
Yes, going NOS 6550s will likely make a notable difference in sound (knowing the amp in question will help folks answer), but the question is whether it is worth the cost. If you want to go NOS, the GEC / Genalex have the best reputation (they were fantastic when I used them in an Air Tight ATM-2 - by far my favorite variant), but in the volumes you are looking at are basically unobtanium as NOS (if you need, say, 4 matched quads, you are looking at easily over $10k, and likely over $15k if you want something that is truly NOS and not just "tests as"). So substantial difference, but probably not worth the price.
Of the 6550s, you may be able to scrounge up 16 NOS Tung Sols in the grey plate variation (which are also nice sounding tubes), but you'd likely be looking at >$6k just for the tubes. Again, 16 Tung Sol black plates (the best sounding variation) are also likely unobtanium - TubeDepot is selling singles for $1k per tube).
In the more "reasonable" cost bracket, you are looking at GE or Sylvania 6550s. That will probably "only" run you ~$3-4k. But the question is whether that cost is worth it over something like the current Russian Gold Lion KT88s in your amp (which tube is better than the JJs, but of more reasonable cost than the NOS options). Or you could look at something like the KR KT88s. The EATs are nice but no longer made and also very hard to find. Even the Yugo type ii Ei KT-90s are getting ridiculously expensive - I have seen quads listed at $1700+.
In short, whether NOS power tubes -when you are talking 16 of them for your amps, are at all worth the difference is highly system and wallet dependent - which you haven't told us anything about. In the abstract, I think even moving from the JJs to the Gold Lions would be a nice little upgrade, and depending on how much difference you hear, you can then determine whether it makes sense to spend the serious coin on NOS.
