So Im running a mobile campaign in the US and I got it to about break-even right now. I have already received a paybump, so I don't think I can really improve much more there (maybe another 10-20% with more volume).
So paybump aside, I need to really focus on improving my funnel. Im hoping to squeeze out maybe an extra 20% somewhere.
Is it just a matter of testing new banners and landing pages?
At this stage do I just keep testing headlines, images, banners etc?
Any strategies/insights you could offer would be appreciated.
One thing to not forget is to test more offers from more networks that could fit into the already optimized funnel. It can make a huge difference.
I can say that even now, the biggest difference we had in getting profitable campaigns was finding the right network to work with; it had little to do with the traffic sources or payout (we already know how to optimize traffic).
Also, be very aware of pay bumps, you might end up with worse results. In my experience, it has become such a must to offer a pay bump that all networks offer it when you ask and then scrub you if your quality and volume don't back out for them.
Always check performance after asking for a pay increase.
Yes, definitely +1 for testing multiple networks. The swing in ROI you can get from doing that is absolutely huge - I've seen +100% before, although that was an extreme case.
Thanks for the feedback guys.
I think the offer I am running though is exclusive to this network, so changing networks is not an option unfortunately lol.
I will definitely maybe test other offers, though this one is converting pretty well so I'd like to somehow make it work. 
The offer may be exclusive, but the vertical is probably not, right?
Also, I've seen networks claim exclusivity before, and it was anything but so make sure you hunt around.
Definitely test more offers, different ones. There are always several offers in the same vertical that are compatible to some extent.
Also, this is overlooked pretty often - many offers have several offer URLs, test them too.
You can also try to play with bidding, try to bid higher to see if the quality improves. Or bid lower but increase cap ... try keyword/category targeting.
Other than that, really testing more banners and LPs and finding the right combinations is the recipe to follow.
All of the tips given above are very good.
The biggest influence will come from testing the same offer on multiple networks or similar offers on multiple networks. Followed by your traffic source and creatives, ie: banners and landers. Another consideration is the geo you are targeting, can this be tested in other geos? That could uncover a huge boost with minimal effort.
As Manu stated, paybumps sometimes are counter productive. It's worth mentioning to your affiliate manager that you aren't sure how much longer you can run this offer, because CPM's and competitive is increasing and your traffic isn't backing out with your CR. I do that from time to time just to "shock" them into adjusting any backend settings they have put in place and definitely have called out the advertiser/network for obvious changes to my CR.
Couple of more ideas with some that are more granular and may not have huge influence, but collectively worth considering...
1. Test more sources similar to your existing traffic source (if applicable). You could see a big swing in ROI but it sounds like you are using mobile display/native/facebook? So not sure the impact but on pops it's worth noting.
2. What is your campaign structure? Try breaking out your most profitable segments into their own campaign. Similar to what matuloo suggested on the keyword/category targeting but perhaps consider being more granular for your biggest placements/winners. Compare banner performance on an individual level with your top performing segments, you may realize certain segments could use a banner that better aligns with the traffic.
3. Make sure your lander speed is optimized to the extreme.
4. I remember reading a case study by caurmen regarding the type of image used and the performance difference of png/jpg/gif etc. (Can't recall the specifics of the post but it was good info and perhaps worth testing).
5. If you're running native or facebook with a multi-page funnel, you could consider looking into a tool that records visitors through your funnel and see what bottlenecks they have or where they drop off.
6. Duplicate your campaign with a lower bid or on another account with a lower bid.
There's a handful of ideas for you. Good luck.
Andrew
Hey JaBong, we have a bunch of US offers, hit me up on Skype @ russelldupzyk91
we can get you rolling on some of our exclusive brands