Searchwater can find contacts significantly smaller than fishing boats, and what matters is the range that can be done at - if your detection range is greater then the legs of your search pattern can be further apart and you will cover a greater area in the same period of time.... coupled with a higher speed I would expect that to translate into a significant difference in surface search capability. Although visual lookout is useful for finalising an exact position (and there are rather more windows, 3 'goldfish bowls' specifically designed for eyeballing seagulls from, on Norman) it's cock all use for actually finding things unless you get really lucky.
The Herc will be far better than nothing, but looking back over quite a lot of SAR flying, ie the actual job done on a callout, the majority seeemed to come in three flavours:
1) Small boat/Lone yachtsman etc lost and discovering a previously unnoticed jacuzzi downstairs, which often involved the Nimrod doing a quick search to locate the nearest surface vessel to conn onto the 'sinkee' to assist.
2) EU fisherman in need of NHS operation to be winched from far SW, Nimrod to provide top cover for helo op at long range - requiring minimum delay finding vessel, conn helo on, provide SAR cover for helo operating outside its comfort zone.
3) Major epic, Piper Alpha type of event - rare, but the C+C from the Nimrod could involve controlling numerous helo and surface vessels, coordinating search areas etc whilst simultaneously handling lots of comms. (Including safety reports from the rescue craft themselves). That kept everyone busy, and the large Nav area was a godsend, I would suggest. It's not about comfort, it's about having a fairly large number of specialists and room to work.
Obviously the Herk can do a search, it can also drop stuff like dinghies, so 'zero' isn't really the case and I doubt BP seriously thinks that - but for capability, for actually doing the job, it is a long long way from as well fitted for the job as the Nimrod.