There's nothing wrong with the T300. I've just had more experience on the g27. I think the T300 has a better feel to it (gives a slightly more realistic feel in the FFB) and the th8a shifter with its build quality and 7th gear is superior to the G27 shifter. I think the T300's FFB feels better because of the single motor driven vs dual motors. I feel the dual motors of the G27 fighting each other for brief moments when FFB is shifted from left to right. Also the belt drives gives a smoother feel over the dual direct gears of the G27. I feel over and under steer with the t300, which I don't really notice with the g27. It's like I can tell that the wheels of the in game car are loosing grip or have to much. People say they don't like the rubber feel of the T300's stock PS4 wheel, but I like the grip, size and options to easily change out the wheel if I ever decide to.
When it comes down to it, I'm a tinker'er and I like playing around with my GIMX setup and trying to evolve it. I started with it running through a HTPC since it was right next to the PS4, but swapping screens back and forth to get it going every time became a PITA, and luckily I have 2 PS4 controllers so I didn't have to go into the menu's and turn the controler off to connect it to GIMX. Then I moved on to a netbook that was lying around so I could have different screens, but then that started to feel like a waste of power and took up too much space. Then I looked into the Raspberry Pi and had found an automated image that a guy named Waveform posted in the GIMX forums. I started with a RPi model B I had lying around and it worked pretty well. Then I wondered if the PS4's usb port put out enough juice to power it, which it does, and I started running the RPi directly from the PS4 via its 2 usb ports (one communicating with the DIY adapter, the other powering the RPi). Then I tried the setup using 1 usb port on the PS4 connecting a usb hub for communications with the DIY adapter and powering the RPi, and that worked. both setups gave me the ability to just turn on the PS4, wait for about 45 seconds, and use the wheel (nice if you only have 1 controller). I got my hands on a RPi 3 a month or so ago, and now GIMX boots up and the wheel is ready to go in about 10 seconds, and it still runs directly from 1 usb port on the PS4. If I can get my hands on a cheap PS4 controller, I might looks into disassembling it so it can fit into a small box with the RPi and DIY adapter.
Racing with a wheel is fun and tinkering around with GIMX is also fun, so I like that I can do both at the same time. My G27 is also mounted to a DIY wheelstand that I made, while my T300 is on a wheelstand that was puchased, so there's another cool feeling of using something else I made there as well. Looking back, like you had mentioned, I should have waited till my G27 was obsolete.