Slow roasting; internal temperature higher than oven temperature

Any increased steam pressure would raise the boiling point, not lower it. However, I doubt that you will be able to seal the pouch well enough to withstand any serious pressure.

What may cause boiling though, is the fact that ovens cycle on and off, especially at low temperatures. The amplitude of the cycles depend on how good your oven is. At 200°F, your pouch will probably experience above-boiling ambient temperatures during the on-part of the cycles. Of course, that doesn't mean that the contents of the pouch is above-boiling, but there may be.

However, I don't think that the braising liquid boiling slightly now and again is going to be a big problem. Sometimes when I've braised meat, there's been some boiling, and the results have still both looked and tasted great.

There's a few things that could be going on here:

• cut: not all steak cooks at the same rate. Really tender cuts cook faster than some of the tougher, more flavorful cuts, increasing cooking time by up to 50%. Cuts like tenderloin, filet, and loin (US) - Sirloin (UK) are more tender and cook faster. Denser cuts like sirloin, top sirloin, and bottom sirloin (US) and rump (UK) cook slower. If you are cooking a denser cut then you simply need to cook it longer.
• Pan heat: Most chefs have really good stoves that produce load of heat, most mere mortals have average stoves which aren't as powerful. Medium heat on a professional, or very high quality stove is hotter than the medium heat on an average stove, so try cranking it up to full blast. Also, make sure your pan is fully up to temp. I cook my steaks on a cast iron skillet, and I let it heat up for 10 minutes before I start frying steak
• Heat contact: even though steak is full of fat it takes some time for this to start working, so coating your steak with a bit of vegetable oil (not olive oil, it burns at high temperatures) will make sure it gets good heat contact

So my advice would be to get the pan hotter and cook it longer. Try adding one more minute per side.

EDIT: @kenny says that he is cooking loin, so assuming it's 3/4", or 2cm thick and looking for medium done-ness I would cook the first side for 5 minutes and then the second side 3 minutes. The uneven times are to make sure it cooks evenly. So you need a total of 8 minutes cooking time. I don't do the flip every minute method because the uneven times work for me, and it lets me do other prep.