I, there is absolutely nothing "proper" about cutting and shutting a seat mount side bracket. Period.
In a heavy frontal impact, or indeed being rear ended, at 3g, a 13 stone body weighs the equivalent of one quarter of a metric tonne.
The steel used in those seat side mounts isn't mild steel, rather it's a carbon steel, and having seen what 15 stone body did to it during a huge shunt into the Armco at the ring in a 997 GT3 RS 3.8 (it had distorted badly, but not failed) it is absolutely imperative that you don't take a hacksaw, grinder or welder to it.
If the OP is struggling to get comfortable in either of the seats he's mentioned, buy the cheaper of the two cars and have one or two Recaro Pole Position FIA/ABE (or an SPG) seat fitted using the correct side mounts and something like the BBi floor adaptor or use the Brey Krause adaptor brackets, but do not start cutting and welding any of the existing Porsche mounting hardware.
When I crashed my GT3 up in Western Scotland I estimate I left the road backwards, in the dark, at 70 + mph. My off road flight (literally) ended when the N/S/R wheel and 1/4 panel hit a tree and snapped it.
The resultant impact cracked two ribs such was the force I was subjected to when the car de-accelerated and my ribcage impacted the seat shell sideways.
The seat and side mounts were the stock leather carbon buckets, and the side mounts were distorted from the impact.
99.9% of the time you may get away with modding the OE seat mounts,but you'd better be sure they're safe come the fateful day when you DO need them to restrain you and the seat, because they will save your life.
Quite frankly what that individual had done to his side mounts on RL can only be described as reckless.
Here's the seat install I carried out on my 1M Coupe :
https://www.1addicts.com/forums/showthread.php?t=833495
This install was a gamechanger, the seats shell did not move millimeter and the driving experience was transformed, such was the increased connection to the car.
Edit to add additional images.
The red arrow in the image below is pointing to the rear crossmember which has been shattered and snapped, leaving the mounting point and bush still attached to the chassis itself (you can see it to the left of the snapped crossmember).