Personally, I am quite weary of the battery clips, I know many people use them without issues but wedging something, regardless of how tight the fit it, on a flying machine seems like a terrible idea to me (I also have already done the mod). It wasn't too hard to solder some leads if you're good with a soldering iron (I recommend buying a thin tip if you don't have one). I have had my mod for over a year now with no issues, if you are handy with wiring and electronics generally, I would say just do the mod yourself, it probably takes an hour on average, maybe two hours really slowly and if you are unsure of your soldering skills and how to take the bird apart.
On the C rating, I would not worry about it too much as you will be running batteries in parallel which basically splits the current load. While you have no telemetry for your auxiliary battery, you will notice less voltage drop on your main battery when using two batteries in parallel than when you use only the main battery. For example, if I go full sticks on just the main battery, the voltage will drop below 4.00v after a few extended full speed runs, but with two batteries, I never drop below 4.00v regardless of how hard I'm flying until the batteries are ~60% depleted.
