The YTR-4335GII is an excellent trumpet and should do everything you need for Grade 8. As always, as you move up our trumpet range (culminating with the Custom Xeno models), you'll find that the response, tonality and overall playability improve.
If your budget allows, we'd suggest the next model up - the YTR-5335G. This model inherits much from the much higher end YTR-6335GII (from our Professional tier) but uses the 2 piece bell from the YTR-4335GII to make it much more affordable. This makes the YTR-5335G superb value for money - and a trumpet that should last you for many, many years.
Otherwise, the YTR-4335GII is still an excellent choice and should serve you well.