I thought it was about a 1/3 less, so for every 2 you upgrade to S&D you can delete one Wood Elf house and maintain the same population - plus gain 40 peeps [that's after you upgrade to levels 24 & 25].
Space wise, since the two [2] new S&D residences take a space of 30 squares [15 each] versus the three [3] that were removed and replaced [2 converted to Wood Elf & 1 just deleted] at 36 squares [12 each] - you can add an extra house for every six [6] you convert. [i.e. 3 spaces saved x 6 conversions + converting 1 more [12 spaces] = 30 available spaces]
In conclusion: [in general...lol] - you can delete 3 wood elf homes, build 6 S&D homes + 1 extra all in the same amount of space.
So DELETE three [3], CONVERT seven [7] all in the same space. [of course orientated differently].
You loose 1200 x 3 peeps for the wood elf homes deleted and convert 7 [7 x 1200], or a total of 12,000 peeps [10 x 1200]; but you gain 7 x 1820 peeps [S&D @ level 25] = 12,740 population.
TOTAL GAIN of 740 extra population in the same space of 10 wood elf homes converted to 7 S&D residences.
Yikes...hope my calculations are right...I wasn't really trying to do a math problem...but it turned out that way....