A sailpack can be used with a loose footed mainsail. It does, however, created drag and turbulence. That's the trade off for the convenience of not using a separate mainsail cover. If you are super concerned about extra drag and turbulence, a sail pack isn't for you. If your boom is up high, and it's difficult to reach it for putting a sail cover over the mainsail, then a sailpack is a wonderful convenience.
As for the sailpack interfering with easing the outhaul: If you install adjustable lazy jacks holding the sail pack up, you can slack them so you can ease the outhaul without the sailpack interfering with the foot.
If you want to get the sailpack out of the way to minimize drag and turbulence for higher performance, you can order the sailpack with include straps/ties that permit you to roll the sides up and strap them to the boom. This usually requires retraceable lazy jacks lines. And it takes more time to roll up the sailpack flaps than using a separate mainsail cover.
Personally, I don't want the extra drag and turbulence. So I have retractable lazy jacks, on my boat, and I use a separate mainsail cover. The main drops right into the lazy jacks and I put the mainsail cover on later. But my boat is small, and I can reach the boom easily to put the cover on.
My Corsair Trimaran's mainsail has 7 full battens and needs a Tides Track (or similar car system) for it to drop quickly and easily. Full battens create a lot more friction than partial battens, so this mainsail requires it, especiallly in high wind.
Dropping the mainsail is nearly idiot proof. It comes down by itself in a couple of seconds in winds up to about 20 kts, as long as I remember to steer above a beam reach and the mainsail is eased. In higher winds, I need to help the last few feet come down with a gentle tug, but I never struggle to control it.
When hoisting the mainsail, the battens don't catch in the lazy jacks as long as I'm on a close reach or higher, and the mainsheet is eased.
(PS. In the video below, the diagonal top batten has been removed, which I need to do to lower the mainsail completely and to fit the mainsail cover.)
Judy B
Semi-retired sailmaker