Is a top heat gas grill, capable of 800°C / 1500°F perfect for pizza and sous vide steak

The machine

I invited a couple of friends who are keen to try sous vide over during the whole process. All 4 of us agreed. The machine is going back.

My immediate first thought was that you had a bad machine. The Vac-Star Sous Vide Chef has excellent reviews on Vac-Star's website and it looks like you are using it within proper specifications. That would be my first hunch, as nothing else seems particularly out of place. If you are able to come back, since this was over a year ago, it would be curious to see if a different machine solved your problems.

_{On a side note, I looked up reviews for Sous Vide Supreme vacuum pouches and they seem good, too.}

Other things

I looked up for reviews of the I noticed the Beyond Salmon blogger had posted a follow-up experiment with sous-vide steaks that I found had useful data. This summarized it well:

What most sous-vide books and websites tell you is that you can't overcook using the sous-vide method. It all depends on how you define "overcook." The steak will not go above the desired temperature no matter how long you hold it in the water bath (well, dah!), but the longer you hold it, the more juice you lose.

I have noticed this from your post:

I have tried various temperatures and times from 55°C to 60°C and from 40 minutes to 4 hours.

Per her experiment, steak A, B and C turned out the best. They also had the lowest temperatures and cooking times:

Steak A: 121°F (49.5°C) water bath for 50 minutes

Steak B: 126°F (52.2°C) water bath for 50 minutes

Steak C: 131°F (55°C) water bath for 100 minutes

It likely would be a good idea to try lower temperatures, and stick to the lower-end of the time scale. Also from the article:

But if you are using an immersion circulator, you might be tempted to put meat in the water bath whenever it's convenient (in the morning before going to work, during kids' nap, etc.) and then have it ready for dinner. It will surely be very tender, but you'd better have a lot of demi-glace handy because it will be dry.

Hope this helps!

130F (approx. 54C) works great. That brings it to the very bottom of the "medium-rare scale", allowing you to sear before presentation without overcooking.

Although, I have to tell you, filet mignon is kind of a waste with sous vide. Not that it won't be great, but a much less expensive cut can be cooked at that temperature much longer and give you amazingly tender results.

The minimum time for a medium-rare filet would be just as long as it takes for the whole steak to reach 130F (half an hour or 45 minutes, depending on the thickness of the steak). Of course you can cook it longer, that's one of the beauties of sous vide, but that won't make it any better.

You can spend a whole bunch less money and get chuck (shoulder) and slice it into steaks. Sous vide it for two days at 130F, and that makes a steak almost as tender as filet, yet more flavorful.

That is my very first chuck steak cooked at 131F for two days, then seared. I used a NASA hot cast-iron pan for the searing, so 1°F less will give you a little bit more time to sear without risking overcooking your filet (which would be really tragic!).

Key Point: As Jefromi says in his answer, medium-rare is a scale. If you do 130F this time, consider carefully as you eat. Is the temperature absolutely perfect for you? Keep track of the temperatures you use every time you sous vide and your impressions at that temperature. Once you know what temperatures are perfect for you, sous vide allows you to duplicate perfection perfectly.