I will give advise based on my personal opinions as I find it quite interesting you are looking for gobs and gobs of power and you noted that you are not a good rider.
First of all, it is not power that takes you to the trophy. It is the ability to handle, control and be the least fatigued rider to help you win that race even if it is a nice ride for the day with friends. It is always better to be able to be relaxed and not always having to handle the machine during everything your doing.
A big powered bike will always be heavy no matter how you look at it. The SXF450 is a great supercross machine but that's about all it is good for no matter what you do or how you gear it. It has gobs of power, is light and rigid and handles great but one problem. It is set up for one purpose only and that's motocross/supercross. All the mods you posted are great but I could almost guarantee a xcw out of the box even in a 250 would take a experienced rider further, faster and at the end of the day still have the ability to do another run
All the guys I ride with went through a bigger is better era a few years back and all on the big Husa's. They have all downsized as the pain from crashing, fatigue from the rides were killing them. I dropped down from a 450 to 350 over the last few years and am even thinking of a 250 in the near future.
When riding a smaller 250/350 that is nice and light, handles well and is in top running condition will leave the big machines behind as the rides will be struggling and working themselves to death while you work the machine with little or no effort.
If you look at the top riders you will find them on 300 2t's and 350 4t's. You can only use so much power until it becomes unmanageable.
Might want to look into a older 500's KTM RFS exc, xc something like that if you really need the power, light and great suspension. Also something to think about is technology if looking for power in a light machine. 08 machine vs 15 machine will be no comparison. My 14 350 has tons more juice than my 06 400 did.
your 08 650 is gonna have about 55/60hp
2015 ktm 350 is about 50hp with exhaust and some tuning and only 215pounds or so. Chances are that your 450 had more hp than the 650 does and is way lighter.
Just some food for thought
Bigger always isn't better, I ride with a guy on a small little trials bike and I can not even think about keeping up to him in the tight single track, only place I got a hope is on the open