Can I convert to FMS Delhi if I score 100 percentile in CAT and do really well in the Extempore and PI?

If you got a perfect score of 100 percentile on the CAT, you're almost there. The reason for this is that FMS has set a very high weightage of 85% for the CAT score in the final selection, so a 100 percentile means you don't need to worry about the other criteria.
However, this 85% does not take your CAT percentile into account. Rather, it considers your CAT scores (not percentile) with the following modifications (the figures represent the weightage given to the specific section):
 

SECTIONS	WEIGHTAGE
VARC	40%
DI/LR	30%
QA	30%

This gives you your overall score. Additional 5 points are awarded to female candidates who appear for the interview when calculating the composite score. 

The following are the weightage in the final section of FMS admission 2023:

Component	Weightage percentage
Personal Interview	15%
Weighted CAT 2022 score	50%
Discussion on Statement of Purpose	10%
Extempore	5%
Marks obtained in Class X	10%
Marks obtained in Class XII	10%
Total	100%