A liner with a hood is more of a fitted sleeping layer than a loose sheet. The Sea to Summit Reactor Midweight Sleeping Bag Liner uses that shape alongside Thermolite Pro technology, which Sea to Summit describes as helping trap and reflect body heat.

I'd compare it as an addition to a bag that already fits the trip, rather than a shortcut around choosing suitable bedding in the first place.

The Specific Features

Thermolite Pro technology in the manufacturer's description.
Hollow-core fibers used in the warmth-focused construction.
Ergonomic shape and fitted hood.

Those details explain the design direction, but they don't justify inventing a temperature increase. Check the current product's documented specifications and how any warmth claims are stated.

The Hood Changes the Fit

A fitted hood can help keep the liner positioned around the head end. It also means the liner's shape needs to work with your sleeping bag's hood and your preferred position. Try the two together rather than treat the liner as an unrelated purchase.

If you turn a lot in your sleep, see whether the added fabric moves comfortably or twists. A useful thermal layer is less appealing if it makes getting in and out unnecessarily awkward.

Is Another Layer Worth Carrying?

For repeated camping or travel, a separate contact layer may also simplify keeping the main bag clean. Read the washing and drying instructions before deciding how that convenience fits your routine.

I'd consider the Midweight when its fitted shape and documented construction address a specific need in an existing sleep system. If your bag is fundamentally unsuitable for the planned conditions, compare the bag itself first. A liner is an accessory to a considered setup, and the right reason to choose this one is its particular fabric and hood design.