We unlock the seekers after level 2, it wastes a ton of time to get another bomb type 'cause the game tells you that you can change the bomb type with the d-pad for like 10 seconds, though you'll get that time back using them in the run, they for example make unlocking the kettles way faster. And you can also use them in the levels for certain strats, since it makes a nice straight ink path which you can follow. And just like the splat bombs, the seeker instantly blows up the zapfish bubbles, which is a convinience. I don't think upgrading the hero shot is that useful, you can just do it if you want to, but it's really hard to say (for now) if it would be faster or not.