Well, we're talking about WW2 tech here, and it's going to be more accurate and a greater rate of fire, plus Nazi Germany's industrial capacity to produce a huge amount of shells.
Did you read what I said ?
V3 themselves didn't do much damages with the few shells they had the time to fire. These weapons were and are still ineffecient, costy, and inaccurate.
From what I read, the only time these things fired, they had the time to send approximetaly 180 shells in a few days to bombard a town in Luxembourg. And they only killed something like 250 civilians.
Is that with such a "terrific" weapon" that you're going to repel any sea invasion ?
A shell like that hitting London would level a whole bloc, and without endangering the Luftwaffe.
These shells weighted only 140 kg, or 310 lbs. Compared to bombs from a heavy bomber that's pretty weak. Clearly. V2 had more explosives, anyway they were not the most effecient weapons of the war and they didn't make UK surrender. Pariser kanonen had almost the same kind of shells, anyway France didn't surrender in 1918, nor sustained heavy losses from the 367 shells fired at Paris, and they were never used against armies, always against towns, because 367 shells are nothing when you want to hit large groups of troops entrenched or dispersed like it was the case in WW1 or WW2.
And when you're saying that these are too far away in time to be compared, just wonder how much progress were done in the field of giant canons between the two wars. Considering these are weapons which were almost never used, you can easily guess there was not a big difference between a Pariser Kanonen and a V3, both had a big range (120km for a Pariser Kanonen vs 150 for a V3), the weight of explosivs was almost the same (140 kg vs 125), only the kind of explosiv used could make a big difference, but considering the canons used in Luxembourg did almost the same loss as the one used in Paris 26 years earlier, I doubt the new explosivs used are revelant.
Now, considering a V3 can fire every 3 minuts, it could be interesting, provided you can really make them fire a shot every 3 minuts. Also provided you can protect them from bombers, because they are huge target. And even during the first world war, were aviation was still pretty weak, the Pariser Kanonen were threatened by planes, and germans had to build a fake canon to protect the real one, and fire other canons at the same time to make sure UK and France would not guess where the real big canon were located (a proof of how much these big things need to be protected).