Wiskie,
I think the best 2 all around upgrades with the least amount of hassle will be
1. Tight bore barrel
2. Hop Up
I think the Madbull tight-bores are great, and use them almost exclusively now. I do have a 550mm Promethius that I had on my M4 Sopmod.
I feel that the simplicity of installation and the immediate benefits make this by far the best bang for buck upgrade for the time and energy it takes to do. I would leave the mechbox internals as they are, and replace things if they break. Installing a FET, although surely having merit, seems like an expensive investment on the chance that your play style might arc a switch. If the ROF is good enough on it now, why open it. Don't fix what isn't broken. I very much respect the investment that guys like Q have made with their guns, and they know what they are talking about, so if you want to go the turbo charged route, then the vets around here are very much the ones to turn to. I am finding though, that upgraded guns become like high end race vehicles...they are great when they run smooth, but you will need to do alot of preventative care and hope you are sponsored by a company that will pay for the expensive repairs.
It will take you less than 10 minutes to do a barrel swap and hop-up bucking/nub replacement...if that.
On My Sopmod I changed the barrel, hop, spring, cylinder, air nozzle, piston head, etc. I was new to the sport and thought I needed to mod the internals to gain that competitive edge, and that's what all the cool kids were doing

I also like to learn about "how things work" when I start new hobbies. It was a great learning experience, but at the end of the day a huge headache, and I regret ever having opened the mechbox. I eventually got it to all work together, and was shooting a very consistent 97 m/s with a 550mm barrel (long!). I probably would have been fine with a barrel and hop swap. If we can find out if the cylinder is ported or not it will give you a better sense of what length inner barrel to get...550 is too long for the CQB
About the twist barrel.
I think it would be very cool to get 4 people together with SOPMODS.
We could install a different inner barrel into each of the 4 guns hop up units and then use them in one gun.
We could get a sense of the accuracy and fps difference in each.
If we had...
1. Stock Sopmod (363mm TM barrel)
2. A madbull 6.03
3. A promethius 6.03
4. Tanio Koba
We could get a fairly good test out of this by putting a bipod on the gun and having consistent shooting conditions, etc.
Anyone have one of the above and want to volunteer for this.