I have always thought this an interesting question.
But lets think about this for a minute.
You are certainly going to get carbide precipitation, but who cares. When you consider the chemistry of 310 I think even with carbide precipitation the Cr will still be considerably higher than the P11. Strength will be proven out in procedure qual. The only other issue would be creep because you are in the time dependant region for those alloys but I do not have a ready answer for that. You can defer to your customer since they are responsible for design and have apparently chosen this transition combination. My opinion would be that creep will be fine but that is just my uninformed opinion. I recommend talking to your customer. You can't avoid the PWHT and they determine the alloys by design consideration. Your responsibility is proving out your qual which does not include creep properties. And having said this if your are communicating with your customer, who is responsible for design, bring up the issue of corrosion as well.
The nickel is a good choice because its thermal expansion is comparable to the P11 and the Ni content will prevent carbon migration.

I aggree wiht what js55 said, however I would alwasy fabricate frm a point of view that you if you dont have to pwht a certain material rather follow a buttering route. PWHT in itself has restriction ito thermal stress on site etc etc. I checked ASME IID and 310H is good up to 816Deg C for Div I design so I dont think youll run into trouble interms of metallurgy. but again taking into account site conditions and thermal stresses generated due to difference in thermal expansion.
cheers
HF