Shoe and pedal change advice

I currently use SPD M520 Mtb Shimano pedals £30, and Shimano Mtb SPD M089 shoes costing £50 and with a stiffness rating of like 3/12, 2 velcro straps and 1 ratchet. They're great for commuting and walking to/from the office. Reasonably comfortable even on longer rides. I cant really complain about them at all, even after 15000 miles or so in them.

Time to upgrade though for performance reasons, to something like the Shimano RX8 gravel shoe, which is £175 carbon 10/12 stiffness etc. But, before I do this! I need your help...

1) will I really experience a lot of difference in power transfer and delivery? What experience will I get with very stiff shoes? Worthwhile upgrade for the money?

2) I ride 98% road, and a little gravel or light trail. I am a road cyclist basically. I like training and trying to improve my ftp, I'd like to do sportive and maybe cat4 sometime down the line. I'm keen right.
So at this upgrade cross-roads, should I instead make a switch to SPD-SL R7000 105 road pedals £75 and Shimano RC7 road shoes £175. There is so much anecdotal 'better power transfer' comments on every article or forum, but is there any study data that shows SL pedals are higher performers on the road than SPD? If I'm going to walk like a plonker on the commute at the office, I'd like the riding to be more enjoyable and/or better performance.
I dont want any other pedals/cleats like Look or Speedplay.
What would you do? What is your experience of SPD vs SL?

Thanks!