I know two very succesful pikers, Chase uses 21/18. I believe Craftybadger uses/used 24/15. Both with heavy-ish armour.
Heirlooming the pike for the +3 pierce is pretty important, but looming armour might be better in the end. The 1 speed increase isn't that useful, it goes from slow to slow.
You might also want to consider using a bamboo spear, but it probably doesn't fit your intended style. It has a lot more speed and more importantly, a overhead. It is a lot shorter though.