Unless you have an overwhelming sense of sentimentality to that bike, I'd be saving for a new bike and just maintaining the current bike.

The only thing that Ultegra will get you is less weight. Shimano 105 5800 shifts every bit as well or better than any Ultegra group of its time or since, IMO. IMHO.

I've got 105 5800 and Ultegra r8000 on two different bikes.