Hi David,
I am now considering buying a new PC with Windows 7 and installing the newest version of CRT Emudriver and GroovyMAME. 
If you're building a new PC, I'd definitely skip Windows 7, we have a brand new deprecated Windows version already: Windows 10.
My question is: Is there an improvement with regards to input lag when updating to the newest version?
Recent GroovyMAME is better at reducing input lag. However, the biggest improvement will be determined by having a faster CPU. The faster the CPU, the lower the latency.
With my setup I have 2 frames lag in Donkey Kong. 
There is a special mame version called shmupmame with reduced input lag and with shmupmame I get 1 frame lag in DK, so 1 frame less then the old GroovyMAME version am still using. Would this reduced lag also be possible with a newer version of GroovyMAME? 
What you're talking about is the internal (built-in) game input response latency, not the input lag itself of your system which is likely higher than that. What shmupmame did is to hack the emulation to remove some of that built-in latency, in part compensating for the otherwise poor overall system response. GroovyMAME's strategy is different, it tries to lower the system's response time to the minimum, while not hacking the emulation.