I have not driven one but have in the past considered the fk8 ctr as well as the FL5, even within reasonable mark-up.
one of the reasons I didn't pull the trigger on the fk8 is its cooling has been known to be a prevalent issue on the track. Aftermarket parts have not known to fully address it. as of yet. By that I mean upgrading the radiator, getting secondary radiator, oil cooler, fmic, aftermarket hood with more vents. I forget whether it was the coolant or the oil that was difficult to heat. Some cars randomly seem worse than others. Mind you this is important only with tracking a car. Maybe on a street driven and/or autoX car its not as significant.
I've not looked into the FL5, but essentially its the same engine. Fingers crossed the cooling is not as flawed. I was hoping to wait a couple of years and let others be the guinea pig before I decide whether or not make that leap of faith. If anyone knows otherwise, please let me know as I really am interested in this car. Unless you haven't already done so, definitely look into some forums. Ive found civicx site to be helpful. Also if you do get this car, please do invest in some anti theft. They do get stolen. If you are looking for jdm aftermarket, check out Blackhawk Japan. Have purchased wedsport wheels, spoon parts for my hondas at much cheaper prices than US vendors. sadly there's no notable discount on Varis aero if thats your thing.
I am not sold either on the GR Corrolla due to overheating rear diff issues, that as far as I know as not yet resolved. If anyone knows different, please let me know.
I do love the 10th gen civic platform. My parents have 10th gen civic touring cvt that I tinkered a bit with. I currently have a 10th gen Si with a mountain of parts but not much installed as of yet. I like the Si because it was a more comfortable alternative to the fiesta and more practical than the fk8 ctr. Much to my disappointment, 3rd and 4th gears are the weak link, but there are aftermarket solutions (I am getting PPG straight cut 1st--4th gears, parts in garage just not installed). I don't need a lot of power to enjoy a car, so being close to stock or around 300whp tuned on 91 (not interested in flex), as an approx goal is good enough for me. I rather get more fuel economy and have it as a fun street and track capable car.
FL5 makes sense over the SI, if you want a huge amount of aftermarket options, better suspension design, and definitely the engine being able to make more power. 11th gen should also be a lot more rigid than 10th gen fk8.
I say if you can, just keep the fiesta. I like having both my Si and Fiesta as they have kinda different personalities.
Fiesta does not really have any strong notable weaknesses maybe other than the tranny with a lot of tracking. I am a bit scared myself as I suspect I am getting some gear whine from tracking the car. cooling is pretty damn good after you throw in aftermarket upgrades. I can lap 20 min sessions 'back to back' in 90f-close to 100f heat without issue.
I am definitely keeping my eye out on this, although I am slightly leaning more on the integra as I like the way it looks a lot more, even if there are some compromises.