After some experimenting, I have noticed a few things:
There are different things to optimize for in a survey.
With Geo survey vessels, if you do not watch them they will all head to the same closest planet and start survey its moons. But efficiency for Geo survey is to have one survey ship survey an entire planet-moon system by itself, so there is no duplication of travel distance.
Same with warp point survey. I have seen them get to the outer ring and the survey ships are leapfrogging each other. You lose the savings of having multiple vessels because they increase their travel distance.
It is very simple to manage 2 ship warp surveys so that they do not overlaps.
But another efficiency concern is the ratio of travel time to survey time. Ideally, you want the ship to be spending at least 30% of its in-system time surveying. Preferably 50%. A smaller ship with the same engines to instruments ratio will spend a larger percentage of time actually surveying.
The numbers I have been running suggest that there are serious efficiency points to both small survey ships and few survey ships per system.
If one pursues that path, you will have to have multiple survey operations going on at the same time, and need multiple survey tenders to check up on them.
There are some disadvantages of course. Multiple concurrent things optimizes for the survey ships being active in surveying more of the time, but each survey takes longer. And as the empire gets bigger the fact that each tender is minding survey operations in multiple systems means that they will be doing a lot of travelling between them, and there is the chance that you will miscalculate and have several survey vessels which are idle for a long time.
Also, while long surveys optimize for reducing the percentage of time spent travelling to new survey locations compared to surveying, a long survey also means a higher chance of ships suffering their last Engineering spare casualty with no tender to report it to.
There is, of course, a major exception to the small size ship paradigm. If you have a +30 Survey officer, you want as big a ship as possible for them. A 4 instrument ship with a +30 officer could be set the task of surveying the close together inner rings of Survey Points while 2 2 instrument ships surveyed the outer ring.
Or for Geo survey, you could have 1 ship rapidly survey a planet moon network and then move on, while Rock-Hoppers with 1 or 2 instruments surveyed the rest.
Note, I think that 1-instrument designs will be less practical in 0.5 because a 1-instrument ship with still have 6 HS budgeted for essential systems, and another 5 HS for armor. (crew, fuel, bridge, spares) 11 HS instead of the current 8 HS for that size ship.
Under 0.6, I think my survey tender will have 5 engines, size 60, have 5 fuel bunkers, escorting size 48 3-instrument survey ships that have 4 engines, same speed. The 4 instrument survey ship with 5 engines comes in at 61 HS, unfortunately, so maybe a slightly larger tender is in order.
On a cost basis, engines start much cheaper than survey instruments, so I believe that survey ships should have more engines than survey instruments until the costs get closer. As I said earlier, the greater percentage of time that a survey ship spends surveying, the better. A higher speed means less time spent travelling, and more surveying.