note on size for enwiki :

That's a +14% increase, I believe that this feature is interesting only for wikipedias (but I can be wrong). We should wait for T144387 to have a better understanding of the current state.

Next step on this ticket is to run some relforge tools to measure the difference between the two approaches.
Small demo for enwiki: http://mw-sug-subpages-relforge.wmflabs.org/w/default_sort_demo.html

Awesome!

Preliminary results on a 10k queries sample:

Baseline: ./relevance/queries/normal/results

Metrics:

Query Count: 10001
Zero Results Rate: 23.8%
Poorly Performing Percentage: 36.9%
Top 3 Sorted Results Differ: 19.5%
Top 3 Unsorted Results Differ: 18.9%
Top 5 Sorted Results Differ: 25.3%
Top 5 Unsorted Results Differ: 24.4%
Top 20 Sorted Results Differ: 31.7%
Top 20 Unsorted Results Differ: 30.3%

Delta: ./relevance/queries/defaultsort/results

Metrics:

Query Count: 10001
   Num TotalHits Changed: μ: 0.05; σ: 0.38; median: 0.00; range: [-1, 8]
   Pct TotalHits Changed: μ: 1.8%; σ: 16.5%; median: 0.0%; range: [-10.00%, 400.00%]
   Charts [ + ]

Zero Results Rate: 23.6% (-0.2%)
Poorly Performing Percentage: 36.4% (-0.4%)
Top 3 Sorted Results Differ: 19.5%
Top 3 Unsorted Results Differ: 18.9%
Top 5 Sorted Results Differ: 25.3%
Top 5 Unsorted Results Differ: 24.4%
   Num Top 5 Results Changed: μ: 0.46; σ: 0.96; median: 0.00; range: [0, 5]
   Pct Top 5 Results Changed: μ: 10.0%; σ: 23.0%; median: 0.0%; range: [0.00%, 400.00%]
   Charts [ + ]

Top 20 Sorted Results Differ: 31.7%
Top 20 Unsorted Results Differ: 30.3%
   Num Top 20 Results Changed: μ: 0.83; σ: 1.65; median: 0.00; range: [0, 10]
   Pct Top 20 Results Changed: μ: 9.6%; σ: 23.1%; median: 0.0%; range: [0.00%, 400.00%]

Default sort had a positive impact on zero result rate, most of the queries that return a result thanks to default sort seem to be interesting:

Not so great new results:

I'll continue to investigate, the difference is huge between the two strategies (30.3% of the queries returned a different set of results)

This is still in progress with no movement since september, is there anything else we want to do here?

Different languages use DEFAULTSORT differently.

Examples:
*Chinese is not a phonetic writing system and there's a tradition on zh.wp to use the first few letters of the phonetic transcription of the title as a sort key. 北京時間 (Pinyin: Beijing Shijian) gets {{DEFAULTSORT:B}}. It would not be useful to use DEFAULTSORT as a keyword for search results because it's an incomplete fragment of the phonetic transcription.
*Japanese uses a diacritic to mark voiced consonants but dictionaries sort topics regardless of the consonant. 動物 (どうぶつ doubutsu) is sorted together with とうふつ toufutsu, but a Japanese speaker with a Japanese keyboard wouldn't search とうふつ expecting to find どうぶつ / 動物.

My general feeling as a multilingual Wikimedian is that the use of DEFAULTSORT is wiki-specific. In many cases where the DEFAULTSORT keys are useful search keys, a redirect would already have been created. I think it would be useful to have the option to turn on DEFAULTSORT as a search metric, but the specific language or wiki must be consulted before assigning non-zero weight to a particular wiki.