Really depends on your traffic usage. I've got a box that is running at 47.24Mb/s on 95% and it's only doing about 4.4TB per month in data usage. The max was 124.15 Mb/s and the ave was 13.75Mb/s.
The few times that it's peaking over 100Mb/s I doubt you'd see much if any slow down on the site. And if your 95% was closure to 100Mb/s then you'd be over your 10TB of data usage anyway. So I would say your fine where you are, bumping it up to 1Gb/s line won't help with your current traffic as it is
The thing to watch for is clipping, where the sustained output is hitting your 100Mb/s for an extended period of time, that is when things get slowed down for your surfers. You'll see the peaks on your mrtg graph start to look more like hills with a flat top versus peaks. This is an mrtg of a cpu, but you get the idea, you can see where it flattens out on the top.
Good luck
