I like where this mod is going ! But this baby worth everything..! So , let the dogs out..
The new coils you have are clearly better , but you have to see how they gone handle the 5-6 Amps that you are aiming to, right? I'm still thinking that the ones hand made will do better , especially that they will give you the possibility to solder its in every position that suits the other changes you want to do ( I have in mind the driver with 3 coils from SRK...) but this is up to you .
The fly back diodes are only 5A so , some Schottky diodes , of 10A or so , will be fine .The problem is that the size will be greater and not fit in the space.., but you have the solution of stacking 3 more SS54 on the existing ones , and here comes an other problem , that you'll have to pair them , in order to have the same voltage drop across , at different currents .Is about the same thing that you've done with the coils , a graphic at 1A , 2A , 3A ...and so on , with the same dropping voltage across the diodes . That means that you'll have to take the ss54 out of the board and match them with other 3 diodes .This has to be done in order to spread the current evenly, between the 2 diodes in parallel.
About the FET , you're right , they are in parallel . The XORB 48 is equivalent with 3400L ,link here: mosfet 3400L 30V 5.8A N-Channel SOT-23-3 Suitable for Use as a Load Switch/in PWM, View mosfet 3400L semiconductor, OGFD Product Details from Shenzhen Goford Electronics Co., Ltd. on Alibaba.com
As you can see , Vdss=30V , Rds on=31mohm@4.5V , 45 mohm@2.5V , 28 mohm@10V , Ids=5.8A .If the data sheet is accurate , the power dissipation on these FETs should be around 0.3W (6 Amps X 0.045 ohm) , that makes them suitable for the driven currents , BUT, not only the Rds on , is the only one that contribute to the power loss , the shape of the pulse on gate is important , the time of transition between the state off-on is also very important. Here , at the transition are the biggest loss of power , dissipated as heat .The FETs are considered fast switched devices , but if the wave form that drives them are not square enough , there is the bottle neck...
There are a lot of very good transistors , with a extremely low Rds on resistance ( ex: TPCA 8028-H , with Rds on of only 2 miliohms !) but the problem is the size or the shape of its , that makes the replacement very difficult. I'm not saying that it is impossible , as far as I saw in your posts , you CAN do it , but , because of the limited space ,hard to do . Maybe , after soldering better FETs , will be necessary to pot the driver , in order to obtain the mechanical shock resistance ...
Now , I've notice that instead of making your work easier , I opened some new paths...
Anyway , the idea is to put the driver on the bench , and test it after each modification made , in order to see the improvements.., so , in order :
New coils , test , double diodes , test , new FETs , test , resistor mod , test.. Also , it is possible that the UI modes to change really hard , after modding...
I hope I didn't harm too much your choices , you are the only one that decide what to do. We can help.. if this could be named "help"... Cheers !