Surge problem at speed

You probably had at least two problems, one of which is fixed.

Ign problem sounds solved. Factory timing figures are conservative because the use of the car is unknown at the time of sale. If your engine 'likes' & runs better with 15* init, doesn't ping, then leave it there. The extra timing could be the result of a large[r] cam fitted previously. It would probably be a good idea to have the dist re-curved with the centri curve reduced if you stick with 15* init.

If a bog occurs when the throttle is opened, it is leanness. Need to know more about the bog. Happens with light or heavy throttle? Just off idle? Or further into the rpm range?