That's pretty much how I got started. I started with 4-5 pound external frame packs and ended up with a 1 lb frame pack that did everything the originals did.

My sequence went something like this.

Cut pieces off aluminum frame
Cut pieces off nylon bag
replaced all buckles and webs with lighter ones
replaced frame with homemade aluminum frame
replaced bag with homemade bag
replaced frame with homemade carbon fiber frame
went to lighter homemade bag (1.9 ounce nylon)
replaced waist belt with lighter homemade belt
changed design of pack
refined, refined, refined
etc.

I've been fiddling with pack designs for about 10 years now. I'm sure I've tried hundreds of different combinations. I enjoy the process.

Have fun!