Heavy and super heavy howitzers were in active development right up till the end of the 50s, with systems such as Oka, Kondensator, but after that were rapidly supplanted by first ballistic and then guided missile technology, as well as the growing effectiveness of fast jet air support.
To keep heavy howitzers as a primary long range fire support option, you would have to avoid the technologies that gave rise to effective missile weapons. The issue with that however is that without those technologies the effectiveness of howitzers will not increase much beyond where it was just post WWII. Certainly the rocket assisted GPS guided shells now becoming available to 155mm pieces would not be possible.
Even avoiding that, it is unlikely that heavy howitzers would be used. 155 has been standardized on because it strikes an optimal balance of mobility, rate of fire and target effect. With modern technology the 155 shell can be just as accurate and nearly as long ranged as guided missile weapons, negating the need for area saturation or very large shell weights. As weapons become more accurate the trend has been for less destructive power, as less is needed to achieve the same effect.
This is the same reasons why western airforces are now beginning to arm warplanes with very small missiles, and have mostly abandoned dumping huge numbers of dumb bombs out of big bombers.
To awnser your question directly though. A modern 8 to 10 inch howitzer with rocket assist and gps guided shells would probably have similar if not slightly better performance to the AGS, the current pinnacle of big gun technology. Which would be a range of around 100-120 miles, and a CEP of about 50m.
However this would be up against cruise missiles which have ten times that range and a CEP a fifth the size, whilst costing not that much more.