First thing about product descriptions for e-commerce sites is to avoid the thin content penalty. The question is how much time or money you want to spend. How many product descriptions are we talking about? And how many of the keywords are on the 2-3 page
If you only have 50-60 product descriptions, after a certain level (200-300 words), your time would be better spent working on building links. Don't waste a lot of time getting those up 500 words, its not going to do much.
Now if you have a 1000+ product descriptions, say if your average description length is 50 words, increasing it to 200 -300 words will have a significant impact on your SERPS, assuming you have a lot of different types of products. Why? Because the probability of you producing long tail keywords increase markedly if you have that many products. The thing with SEO is there are a lot of searches that produce than 100 searches a month. The longer your descriptions on a lot of different products increases your chances of getting those long tail keywords in the first page of the SERPS or even rank number one. The problem is a lot of these long tail keywords you won't be know until you actually get traffic from them. Some online tools help, but they aren't accurate when it comes to keywords with low search volumes.