Seems your bus is at 20.30 from kalasipalayam 21.15 from Bommanahalli . You have more options to reach Kalasipalayam/ Wilson garden than Bmmanahalli from airport. IMHO take the first bus to any of these locations from BIAL and inform SRS by calling the BP change.
Please note that the buses to Ecity from BIAL takes outer ring road and others take inside city routes. Traffic will be more or less same in all the routes in the said time.