First of all, you should have calibrated the tip, the default values will be wrong, I can't make a perfect calibration for all boards.
The default values between T12 and 245 are almost identical so it shoudn't affect the behavior much.
That is the point

it was showing 400C because i used some of my T12 profiles on C245 tip, it was just a test.. to see how it behave, on C245 profile i calibrated the C245 tip correctly that you can see on my first video.
Oscillation is not due the PWM, but a combination of PID+filtering settings.
........
This is mainly caused by Kp, try reducing it to 15 or even less, but that tip will never perform well.
Also increase the ADC sampling frequency, go to IRON, ADC Time=100mS, Delay=10ms, PWM Mul=5, will help a bit to keep in under control.
I was thinking same initially.. but T12 profile and C245 have exact same PID settings, only thing that changes actually is Impedance nad Power setting.
I tried setting Kp as you suggested but that changes nothing, I think that is not PID related, and it something in power limit function. I did some more test,
not sure how limit is calculated but my observations are quite interesting.
ADC Time=200mS, Delay=20ms, PWM Mul=1, Impedance= 2.5Ohm Power=100W: huge
fuluctiations current between 3-6A
ADC Time=200mS, Delay=20ms, PWM Mul=1, Impedance= 2.5Ohm Power=
200W: flat current 6A (144W)
ADC Time=200mS, Delay=20ms, PWM Mul=1, Impedance=
10Ohm Power=100W: flat current
7A (
168W)
ADC Time=100mS, Delay=10ms, PWM Mul=5, Impedance= 2.5Ohm Power=100W: flat current
3A (
75W)
ADC Time=100mS, Delay=10ms, PWM Mul=5, Impedance= 2.5Ohm Power=
200W: flat current 6A (144W)
ADC Time=100mS, Delay=10ms, PWM Mul=5, Impedance=
10Ohm Power=
200W: flat current 6A (144W)
ADC Time=100mS, Delay=10ms, PWM Mul=5, Impedance= 2.5Ohm Power=
250W: flat current 6A (144W)
ADC Time=100mS, Delay=10ms, PWM Mul=5, Impedance=
10Ohm Power=
250W: flat current 6A (144W)
Btw my C245 has exact 2.5 Ohm heaters.
But! That tip will never work properly.
And I dare to say it will fail very soon, I recall buying few clones some years back and all failed within the first week or the tips fell off.
Let's see :-) one of my friend is using clone tips for 2 years already without any issue, an mine surviverd my multiple "water tests" so i guess it's depends on your luck
I'm curious to see how the original firmware performs in your testing rig, give it a try! 
I soldered original STM instead MM, so it will too much effort to re-solder it back just for test