Of course a better projector and a better screen will help. But, cost effective is not typically part of those things.
Realistically, since no information was provided it's impossible to say what upgrades may may make sense for this setup. But, a free screen is not likely to be seriously outperformed by a $1,000 screen. Yet, there are some black screens that cost $3,000+ that will perform great.
Yet, if you are using a 1,000 lumen projector, then it may make more sense to go with an upgrade on that before upgrading the screen.
Still, several thousand dollars, or more, is where I would set the price point for any upgrades if you want them to be of any real significance. (IMO)
Add to that - the 20' distance to the screen is a long ways to throw an image or it may be producing an image that is far to large for the room/environment.
Generally speaking, in a business environement, with typical boardroom level lighting (flouescent) you want a projector to be rated to deliver about 60 lumen per square foot of screen surface area. So, a 6x8 screen = 48 square feet and should have about a 3,000 lumen projector. Real world projector output is typically 10-15% less than rated output, which is already factored in.