I have not used "line-X" brand but I used used some spray on "Boom Matt" in my fender wells. It helps some but if you want the best results you want two layers of stick on insulation (IMO). Lots of threads on this.... do some searching.
Something like the Dynamat (asphalt base) to deaden sound and it has a thick aluminum backing, then follow up with what I call "Fat Matt" (but others make this product as well). Its 3/4 inch thick soft foam with thick aluminum backing. I used this on my fire wall, under my dash and under the hood extensively. It cut out all of the heat transfer and cut the sound levels down dramaticly.
But it would not hurt to spay a liner on (if it dont cost too much) and have three layers of deading.
2017 Axis / 25.4: Big Foot Levelers, Aims 1200 Watt inverter, Ceramic Window Tint, Full River AGM 6V batteries, Front Hellwig Sway bar / Rear Track Arm, Safe T Plus Steering Stabilizer, PD 4655 converter, Hard Wired Surge Guard, WEboost + WiFi Ranger, LED Headlights/Driving lights, Fresh water surge tank, Black Tank Tornado flush, 4999 zip ties.