Its sounds quite a lot to me, I usual put 80 - 120 (tops) for a 40 pint batch (23.5litre), however if 9 was OK another 1.7 is quite a small amount. Keep an eye on the platic bottles when they have become very firm, put everything somewhere cold that will help stop it going to far.
I did some cider where I put to much primer wanted to make it quite fizzy, I think I put about 180g maybe 200g in a 25l batch, they havent exploded but fizz like warm champagne and come out the top when opened. Only way to slow/stop it is to serve really really cold.
I wouldnt worry to much but after a week or so for the secondary ferment to do its bit shove em somewhere cold and safe so if they did go it wouldnt matter! Or give em two weeks and drink the lot quickly
