After the Slater Koekkoek call up from the Syracuse Crunch, the Lightning are $1.82 million under the upper limit of the NHL salary cap. That equals out to a $5.8m AAV contract in cap room. The team is not getting close enough to the cap to put Jason Garrison on LTIR to gain any benefit. The Lightning also doesn't have the contracts in the AHL to be able to manipulate the cap enough to get there. They'd have to have a guy or two go on IR to be able to call up enough players to make a LTIR declaration and it just wouldn't save that much cap room, so not likely to happen or be needed for that matter.
If the club maintains the roster composition of the moment until the 2016 NHL deadline they'd have the equivalent of $8.2m AAV cap room for trades. I figure they want to save at least $1.5m for injury recalls. So they do have a pretty good flexibility in being able to take on a contract at the deadline, be it a rental player or otherwise.