It may void your insurance cover. Or not.
I'd expect it to work well enough.
It may not.
A 5W LED lamp may produce 500 lumen (100 l/W)
A 6W one should.
A 7W one almost certainly will./
I'd look at the illumination patterns of LED and Halogen bulbs.
The Halogen radiates upwards and downwards, with the upwards light reflected downwards.
Depending on design an LED bulb may have all light radiated in one direction - and the radiation angle may in any case be smaller than for a Halogen lamp.
Comparing actual results may prove useful.
A Halogen lamp will reduce in output over hundreds to a few thousand hours.
A reputable source LED lamp should have close top constant apparent output for 50,000+ hours.
The human eye/brain does not readily distinguish differences of illumination of perhaps 50%. All else being equal - if you have a say 500 lumen and 350 lumen lamp, if each is viewed independently, almost nobody could tell which was which.
In most cases lamps of that type have the 12V supply voltage present on the two uninsulated arms. This is a potential fire hazard even when limited energy power supplies are used. If the power supply is capable of supplying current well in excess of the lamp requirement the fire risk will be higher. This may invalidate your insurance cover, regardless of bulb wattage. A fuse (or better) is recommended.
I have so far been able to find a non-paywalled version of this paper, which describes the hazards due to low voltage low energy sources.
Vicars, Richard. Small, James. Munson, Terry. Parrish, Christopher.
Low Voltage: The Incompetent Ignition Source Dispelling the Myth. (2010).