No, I predict it will be worse when through running onto Thameslink starts.
What GN is seeing is basically the transition to Thameslink standards of performance. Which is one reason why many people are sceptical about the whole concept of mixing Thameslink with GN.
I started out believing that Thameslink performance was always much worse than GN and was quite surprised to see that back in 2010/2011, even Thameslink performance was routinely getting PPM scores better than 90%, except perhaps around leaf fall season.
Both routes have worsened very considerably since. GN's decline had reached the point where my perception was that Thameslink performance was now better than GN! Although this was true of the 4 weeks to 15 October 2016, the actual data shows that period to be the exception. Thameslink performance was a fair bit worse even than GN's dire performance for the 4 weeks to 10 December, with a PPM score of 60.1%.
I only use the Thameslink core and this probably explains my perception of better performance than the data shows, as I tend to be much less disrupted by the odd delay or cancellation than a passenger needing a specific Thameslink train.