SELECT MAX("Laps") FROM table_45541 WHERE "Team" = 'walker racing' AND "Grid" > '15'
What was the highest lap count for Walker Racing with a grid larger than 15?
CREATE TABLE table_45541 (
    "Driver" text,
    "Team" text,
    "Laps" real,
    "Time/Retired" text,
    "Grid" real,
    "Points" real
)