When talking about the Mosquito as a replacement for the B-17, are we talking about the normal Mosquito with 2,000 lb bomb load or the bulged bomb bay Mosquito that could carry the 4,000 lb HC bomb? (What was the degree of performance reduction caused by said bulged bomb bay? I presume it wasn't that much.)
We are talking about both.
The bulged bomb bay only enters the equation from early 1944.
I'm not sure that I have ever seen what the performance difference was between the standard and bulged bomb bay Mosquitoes, and it is likely irrelevant because the bulk of those with the bulged bomb bay were Mk.IXs and Mk.XVIs, with only a few of the IV/XX so converted and used operationally.
The Mk IX and XVI had the more powerful Merlin 70-series engines, so the performance improved despite carrying a bigger load and having a bigger belly.
The issue with replacing the four engine bombers with the Mosquito is the issue of striking power. The B-17 regularly hauled 5,000 to 6,000 lbs of bombs to most targets; that's 2.5 to 3 times that of the regular Mosquito, and 1.25 to 1.5 times more than the bulged bomb bay version. Which means to replace the striking power of 100 B-17s would require 125 to 150 Mosquitos with bulged bomb bays, or 250 to 300 normal bomb bay Mosquitos.
The argument is not as simple as that.
1 on 1 the Mosquito doesn't have the striking power, in sheer weight of bombs. But with the bulged bomb bay they were able to use more destructive bombs - the 4,000lb High Capacity (the cookie) and Medium Capacity bombs. The B-17 could not carry these weapons internally with the standard bomb racks, and I am unsure as to whether the bomb racks could be replaced with a single point rack, as was done for the B-29 to carry the nuclear weapons. The British 4,000lb bombs wouldn't fit between the bulkheads, not sure about the US AN-M56 4,000lb Light Case bomb.
I also doubt that an individual Mosquito could bomb as accurately as a B-17 from altitude. The Mosquito is not as stable an aircraft.
And the 8th AF bombers were quite accurate in early missions by each aircraft bombing individually. However, this put the bomber formation under more danger, since it took some time for all to bomb. The formation bombing tactics were devised to minimize time over target and provide better mutual defensive fire. The downside is that the bombing became less accurate, as it was spread over the width and length of the formation.
The question then becomes whether Mosquitoes bombing individually can get more bombs on target than a formation of B-17s. The second question is whether a smaller number of bigger bombs on target will do more or less damage than a larger number of small bombs.
The Mosquito would also have more options in bombing altitude. They could improve accuracy by bombing at lower levels, down to ground level, if need be. Closer targets can be reached by flying at near ground level the whole trip, possibly avoiding or delaying detection by radar.
And if enough crew are available, it would be possible to run a couple of missions in a day with each Mosquito.