Axle to crown on the Lyrik is 545mm. I'm not 100% sure of the a2c on the Thors but a Fox 32 150 is 520mm so likely the Thor is at least that if not lower. You could be adding upwards of 25mm to your front end if you run the Lyriks at full height, which would knock a degree off your head angle and raise your BB slightly. You will certainly notice that and you may find the 5-spot is not warrantied for that amount of fork travel.
The weight increase is significant; around 1.5lbs or c.700g. You will notice the weight as it's a pretty significant gain; whether that bothers you or not is a different question and would depend on the overall weight of the bike as it currently stands. I guess if it's already well under 30lbs then probably it will make little difference, but I find that every extra pound over 30lbs seems to feel a lot heavier (if that makes any sense).
The height issue can be adjusted by the coil u-turn facility so then it comes down to stiffness and damping as benefits for the change. For riding big country trails I am sure you will feel the added stifness and I imagine the damping will be better on the Lyriks (I run a set myself but not on a 5-spot), but I've only ridden the Thors once. The Lyriks damping is great and very adjustable. The gate feature does work well and allows you to stiffen the front end up but set an adjustable level at which that platform will 'blow off'. It's not something I use a lot but it does have its place.
There are a set of new coil u-turn Lyriks at Freeborn for £400 I believe. Call them and check as that's a bargain price – 01403 251 252