If I get mastery enhancements then I would master Observation Haki

Mastery enhancement merely allows one to increase the quantity of existing mastery, rather than advance the quality of mastery, allowing for the stronger, more efficient use of abilities, rather than an ability’s unprecedented evolution.

So I get observation AFTER mastery boost? Or at the same time?

Technique enhancement does not grant evolved technique. It merely allows more proficiency at currently mastered skills.