I've been contemplating an upgrade to an "E" system since late 2013 -- first thinking of IB-E, then Haswell-E. I keep putting it off.
Other members have remarked to me that they expect to get another four years service from a 2600K or 2700K sporting overclocks between 4.6 and 4.8. And I see that the OP only has the 2500K overclocked to 4.2Ghz, while I think it MAY be capable of 4.5+ without pushing the full-load turbo-voltage any higher than 1.37V.
I have a close friend who is crippled with MS, and hasn't been able to effectively use his computers for about two years now. In his frustration, when I begin to discuss computers during our visits, he asks me "But Bonzai -- WHAT do you USE your computers FOR?!"
So, with little difference from my own situation, the OP could "tweak and wait," or spend the money. But that question in the above paragraph has a lot to do with stocks and flows of money together with just plain "satisfaction."