Hi. Yes, definitely increase your time. I have mine set for 5 seconds and I seldom lose an auction. Sometimes people think 5 seconds is too much, but it never fails ME!
The sellers have no way of knowing that you're set up to snipe. Actually, I don't think they care. They just want to sell their stuff. As far as they are concerned, you are just 1 or more of the number of visitors on their counter.
IF, however, you place a bid on eBay first, they will know you are in the ball game. Why would you bid on eBay first? There's no reason to. Do it all through AucSnip and you'll win auctions. (Unless, of course, you are sniped by another sniper. But that rarely happens to me.)
Recently I sniped item 160158471626. Just for the heck of it, I decided to modify my highest bid on AS. When I saw it prior to the end, the going price was $40ish. Then all of a sudden someone bid $160.00. Then I promply sniped it at $161.61. If that other person had only bid $110, my snipe wouldn't have been so much.
So, just put the highest amount you are willing to pay and have it set for 5 seconds.
Good luck............Nancy