بسم الله الرحمن الرحيم
(1) There is no special virtue recorded in the Hadith about keeping the fast of 27th of Rajab and offering nafl salah.
(2) First of all, there is difference in the date of Mairaj Night and further there is no proof of special worship in this night from the time of holy Prophet (may peace and blessings of Allah be upon him), Sahabah and Salaf Salihin.
Allah knows Best!
Darul Uloom Deoband