If I understand you correctly, you want to complete the CFI written and flight check ride before you complete your comm.
Two things, number one is I dont think you are allowed by FAR's to do it, I dont have a FAR/ AIM with me but am sure you need a commercial certificate (even though you are not going to instruct untill you have a comm).
Second, the FAA CFI if done with a FSDO is probably the hardest and most comprehensive flight test around. Your chances of passing without the knowledge aqquired during your comm are practically 0%. You can look forward to a 5-8 hour intensive oral question session, in which the examiner gives you very little room for indecision or lack of knowledge.