Your light is huge for that size haha. But if you can keep the temps under control, light it up! Just beware when you stick a 600w HPS in there (assuming you have a MH atm) it is going to get toasty! As for not feeling much suction, you never do via a carbon filter, but it is there. It will reduce a bit with a filter but not too much.
The one major problem you have is your humidity (and your tent getting sucked in, but read on). Now i am going to take a wild guess that you are testing this without having your plants in there. If that is the case, your humidity should rise quite a bit. If not, and that is the reading you are getting with your plants in there and the system fully running, you need to humidify that box and fast. Your humidity is optimal at around 50%, and should never get below 40%. While vegging, your humidity can rarely get too high, however while flowering, naturally your humidity rises and you want to keep that below 55-60% to avoid bud mould.
Buy a small humidifier if it is still pretty low, and put it the opposite corner to where your carbon filter is. This way the mist will get sucked across your plants. Keep it on and monitor the RH every half hour for a few hours. If it starts to get too high, put it on a timer to go on and off every 15 minutes. Then monitor that, and adjust the timer giving longer periods of switching off to find the "sweet spot" where your RH is what you want.
Keep us updated man, that much light in a tiny box should produce some intense buds! Just make sure everything else is just as abundant, as your plants will grow only as well as it's weakest supply.
Sounds like your air exchange is high enough, however don't let your tent get "sucked in". It means your fan is struggling and will eventually break it. Open up all your vents and let the tent take its natural shape. Just remember if your tent is concave, the mylar lining isn't doing its job properly, as the bottom half isn't getting as much light.
So keep those babies well fed, and flower fairly early, as they will grow 3 times as fast in flowering compared to vegging and will stretch for the light. This might be a good time to look at ScrOG (Screen Of Green). Your setup sounds perfect for it.