The bowie is absolutely not the best boss killer. I don't know if it is the worst though because it happens in total safety, it's just a little slow. A properly built bowie has a strong Valk, that can easily stand toe to toe with Diablo, or whatever else. It may occasionally die, but it's a quick resummon.
If you want your merc to survive against Diablo the only real choice for a polearm is Kelpie's Snare.
Simply put, the bowie is a safety class, it is not the fastest killer at /p8 by any stretch of the imagination, but neither is the summoner. Hell, the summoner is way, way worse. However, it's a bit of a strategic class, positioning is much more important that with almost every other class, but you get a decoy, valk and merc to keep you safe and herd monsters. I am positive that a well build barb can blaze through the Pit faster than a Bowie, but it's more about fun than efficacy. I cannot play a Barb, I hate them, they are boring to me, so I run the pit with my bowie, Windforce and Witchy on switch. It doesn't take long, I have never come close to dying and I find plenty of great stuff, she also often runs the WSK levels, depending on mobs, I may run my Java through there instead.
I agree, the bowie is not the fastest killer at higher MP levels, but to me, and others, it's one of the most fun.
I have two necros, because of the way I level them, they get to level 70 very quickly. I only play the boner, the summoner is, well, not slow against regular mobs, but crap against multiple boss packs, and act bosses. My boner just kills...well, everything, but has 1 row of health, 1 of full rejeuvs and 2 manas, and I still have mana issues lol. I could probably use some better gear and my poor merc is forced to use Insight. I am going to try an Insight Iron Golem as well. That should solve the issues. He will be deadly in WSK.
Don't get to down on bowies, they are super versatile, can kill everything in the game with no assistance (a la Barb), and are fun to play but yes, not as quick as a whirly barb, but then, whirly barbs were not so fun to play during the age of Iron Maiden now were they?