i think you are asking for an opinion on those that have spent money on either new speakers or an external amp.
external amp on stock speakers outperforms a speaker swap simply because the biggest limitation of a stock system is the weak amplification. the stock speakers, while cheap, are well integrated and sealed to the vehicle. midrange clarity will suffer but overall sound quality improves with better amplification.
i've tried both over the years in several vehicles. a better amp wins almost every time. if the stock speakers are blown, nothing will help them. if the stock speakers are more than 15-20 years old - they **** no matter what.