Well, it is going to be your decision at the end of the day.
The CATs are a good resource to 'improve' your weaknesses. Don't pre-empt what is going to happen to you on the D-day based on your MGMAT/Kaplan
etc CAT scores. Everyone reacts to a non-official source differently. Obviously, that is not the objective answer you were looking for. But, as with many other things the answer is in fact 'it depends'.
I may help you with the general trends though:
1.) It is not very difficult to have a 30-40 point jump from MGMAT CAT on the D-day. I've seen many people do it (including myself). Especially if the 680 were from the first few MGMAT CATs. (1-3 out of 6)
2.) GMAT prep is the best indicator of how you are doing. I took 2 GMAT preps going into my exam. I got 760 on both and 770 on the real thing.
3.) MGMAT quant is definitely harder than the real thing. Also, the MGMAT CATs have been re-calibrated. So comparing your performance to your performance from last year doesn't make a lot of sense.
If I were you, I will take a call on whether I can improve substantially in the next few weeks (judging by performance/comfort/exhaustion/topics covered/weaknesses etc.). If not, I will proceed to take the test. I think it is pretty doable.
Lastly, if you do decide to take the test, ask your brain to shut out on these scores and the difficulty levels. All that matters is those 37 questions and 75 minutes; And belief goes a long way.