*looks at two pre-ban Tanakas and one post-ban Tanaka that are my main bolt action rifles, and Bell 1911 that still gets regular use*
So uhhhhhh... I think the pre-ban Tanakas are probably about 25 years old or so. The Bell 1911 is the one that has been in specifically my collection the longest, as I bought it new 19 years ago from a shop in Almeria in Spain, where the Tanakas I acquired from other people more recently.
You can definitely tell that the Tanakas are an old design, especially with the weird stock hop rubbers, and they do require a bit of fiddling to get them running well, but honestly the difference a fully upgraded Tanaka m700 vs a fully upgraded TAC41 is absolutely minimal. Is there a difference? Yeah, my SRS for example is more consistent than my Tanakas, but it's really not noticeable in game. Accuracy between the platforms is pretty much the same, it's just the slight power inconsistencies due to the way green gas works, but even then my Tanakas tend to get about a 0.1J variance at worst so all shots would be between 2.2 - 2.3J for example, where a spring rifle you could probably narrow that down to 2.25 - 2.3J as a range. If you HPA'd it, the difference would disappear.
However, the difference in getting an old platform up to that level of performance versus new platforms is an insane gap. With a TAC41, you can get that out of the box, slap a Rapax 2J spring in it and you're good to go. With a Tanaka m700, you have to replace so many of the internals, often with parts that are either sold out everywhere or have to be sourced from some obscure shop in Asia with a terrible website, and then need to have the technical knowledge to install them yourself, because if you find a tech who is willing to do it for you, you won't have the knowledge to then maintain that rifle and use it all through the year. TAC41 you can just shove that 2J spring in and then use it until it breaks. Tanaka m700 requires constant maintenance and care; I used mine on Sunday and today I'm taking it apart for a clean and relube because I think there was some sand still in one of my pouches that got into two of the mags and then got into the workings of the rifle...
And the Bell 1911 still performs fantastically well, but only because it's a TM spec 1911. All those lovely fancy hi-capa internals slot right in, so it's got a lightweight blowback unit, fancy hop unit and rubber, a nice barrel, stronger recoil spring, nicer nozzle and return spring, nineball gas routers in the mags... It's an absolute beast of a pistol because the design for 1911s / hi-capas haven't changed from the old TMs, and therefore all the modern parts fit into it. If I kept it with the stock parts, it flat out didn't function in the UK climate unless I stuck red gas in it pretty much all year round (during the heatwave I might've gotten away with green)
I think if you got a really old gun and kept it with the weird old parts then yeah, performance would be absolutely wank. Back when I first started airsoft, if your AEG hit 40m you were the absolute king of the 1.1J 350fps crowd (back then, if chrono happened, it was on fps because everyone was slinging .2s anyway, or .25s with an undefined lower fps limit at most if you were super fancy), and if your 500fps sniper rifle (again, .2s or at most .25s) could hit 60m that was considered some sort of freak of nature. But if you have the patience and willingness to learn about it, old guns can often be made to perform on par with newer guns, but will require a lot of newer parts to do so anyway.