Upgrade my tone arm?

Matching arm to cartridge is extremely important. Who or what gave you the idea the Triplanar would be better suited to the Miyajima, and why (i.e. make sure you’re not just going on price)? The Shilabe seems to be very low compliance, and the exchangeable headshells of the Ortofon would allow you the flexibility to add some extra mass if needed (e.g. 20g headshell gets you up to 24g effective mass there, vs. a fixed 14g on the Triplanar). You should definitely solicit opinions from Miyajima users, and what arms they like. But that Ortofon looks pretty nice as it stands - premium/custom Jelco build, dynamic balance, headshell flexibility, 12"...

Even if the new arm is higher quality in build, materials, tolerances, and overall design, it could all be negated if you're moving from a better marriage to a lesser one for the cartridge in question. 

A significantly better arm is a big improvement, just the same as a significantly better turntable, cartridge, phono stage, shelf, speaker, amp, IC, PC, conditioner, anything. Dang I coulda said anything right up front and saved all those pixels. Oh well. Not like the internet is running short or anything.

mulveling is right. In the case of the Miyajima you are not likely to notice much difference. The Triplanar is a really nice arm and certainly with more compliant cartridges like the Lyras and Clearaudios you will notice a difference. If you want to make the jump into hyperspace check out the Schroder LT. 

You should consider a conversation with Tri at Triplaner- it’s a tight community and while I have not heard of your cart on that arm ( I am assuming you are looking at the new 12” carbon fiber ? ) , if somebody is running it Tri will most likely know.
and audio is a system especially transducers and dynamic components- so “ better “ is a myopic simple viewpoint. And yes, I own a Triplaner and a Lyra a known combination that yields excellent results.

enjoy your search @mulveling  gave you excellent advice

The Ortofon RS-309D is a very good tonearm. That being said I just upgraded to an Ikeda IT 407 CR1. It was a distinct upgrade from the Ortofon. Currently using it on my Garrard 301 in a Woodsong plinth with an Ikeda cartridge.

I have not owned a Triplaner tonearm so I can't help with any thoughts on that.