2nd outing with my new G9II: Love the detail! Upgrade to 300F4?

It's been a long while since I was spending my times in the parks, forests, wetlands etc. Just to busy with the job.. but the last few weeks I decided to spend time on this 'old' hobby again. I bought the new G9II and went to one of my favorite parks.

I'm very happy with the image quality I get with this combo. M43 has come a long way. I remember when I shot with my 'canon 20D' and would never go past ISO1600. Now I get shots way higher on a smaller sensor and they clean up like clockwork. I love the new AI-implementation of the noise-removal tool in ACR together with the ability to use masks on the RAW files. I use to to selectively sharpen the subject and selectively soften/remove noise from the background.

Before I started shooting again, I thought to myself that if I would like it a lot.. I might upgrade to a Full-frame setup. Seeing these files I'm just not so sure anymore. I might invest in a olympus 300mm f4 with a 1.4 TC and a 2.0 TC instead. I would love to hear if this lens works well on a G9II. Also.. I would love to hear how the IQ is with a 2X converter. In the forest and parks I'm ok with 420mm or 300 @F4 when there is not a lot of light. In the fields with good light I would love 600mm. Any thoughts?

When looking at the last 3 shots, I can't imagine a lens being much sharper.. nor do I need it to be. Would the olympus 300F4 be able to deliver images like this @600mm and f8?

Troglodytes troglodytes

400mm, F/7.1, 1/250 ISO1000

Sylvia atricapilla

400mm, F/7.1, 1/200 ISO1250

Dendrocopos major

400mm, F/6.3, 1/320 ISO2500

Erithacus rubecula

400mm, F/7.1, 1/200 ISO1600

Erithacus rubecula

400mm, F/6.3, 1/200 ISO2000

Prunella modularis

400mm, F/7.1, 1/320 ISO 640