I was considering selling my relativley new laptop for a Maxwell Gpu

depends how much it would cost you. if you could just refund your laptop then easy, but if you dropped say $400 or more then its definitely not worth it.

I agree

if you can sell your laptop and get the new GPU for a few bucks then why not!, if it's too expensive you better don't. I would apply some math though, what about this:

P is the price you paid for it
Y are the years you plan on having it for
D is difference which equals P-R each year, let R be reselling price in 1 year, it can also mean money spent per year

If P/Y = D = P - R , then it may be worth it to do the same process yearly, let's say you're planning on 5 years, 1500$ budget, and reselling for 1200$ after 1 year
P/Y = 1500$/5Y = 300$/Y = 1500$-1200$
You'll have spent the exact same money if you were to buy one today and one in 5 years but you wouldn't have spent 5 years with one laptop but 5 newly bought laptops in 5 years (5 new generation GPUs). However, I doubt you'd sell your laptop for 1200$, 900$ seems a reasonable price.
If the P-R Difference is worth it for you (even though it may be higher than Y*D) then it's worth it, but that's an objective matter

Remember Y e R can be any n e R >= 1 (or even months, but 1 year and over makes more sense as GPUs/CPUs are usually released yearly), therefore, changing Y won't make it worth it unless your budget suddenly increases/decreases, and this only works if you are sure 100% you'd have enough money which we never are, and assumming you'd want to spend P twice, this is mostly hypothetical

edit: Some grammar

tldr; I would only do it if the 30% perfomance increase was worth the money