Seatec Gabbiano..extra power?

Hi guys, I don't post often here but as I've been spearfishing in ireland quite a lot this summer I'll have a bit more knowledge and also questions to share with the boards.
First, a question.
I currently have 2 guns..a Seatec Gabbiano 77 (open muzzle)with one 16mm rubber and also a Beuchat mundial 60(closed muzzle) also with a 16mm single rubbers.

When the via isn't great I take out the beuchat but most of the time I use the seatec. I can't fault either gun for accuracy, I've got good aim but I can't take all the credit.

I'm just wondering is there a way for me to add power to the gabbiano using an extra rubber or another way? Sometimes I feel the spear could do with an extra speed boost. I'm not using a reel so my range is limited by the line which I have doubled up. There's no space to add an extra rubber as the one on it is connected using the proprietary seatec system but I did find thishttp://sportsbutikken.dk/catalog/product_info.php?manufacturers_id=58&products_id=3925

Just wondering if its worth my while or should I upgrade to a longer gun with double rubbers. Bear in mind it's irish waters, not always good vis.

Thanks for any help,
Marco

That does seem slightly overkill considering how relatively short my line is compared to a reel setup. She's accurate as she is so maybe I won't mess with it until i'm ready to upgrade to a bigger gun if ever!
Thanks anyway