Cheap carbon wheels? 50mm carbon rims from china on some light hubs would probably save most of that weight gain and do a lot for the aerodynamics.
It might not just be a case of more weight, but where it is too. There was an interview with Cav’s mechanic talking about Cav swapping to Di2, it wasn’t so much the weight that persuade him (at the time Di2 was heavier), but it meant all the weight was around the downtube/BB, which meant Cav could throw the bike arround more in a sprint as it felt lighter than having mechanical shifters on the bars. I imagine hydraulic shifters aren’t light? In which case a new bar and stem might offset that?
Depends what you have and what the budget is:
£4 – new bar tape (always good for a feel good factor, stick something racey and thin on)
£40 – new tyres
£400 – Lightbike wheelset
Another £400 – new finishing kit, saddle, and a pair of racey carbon shoes.