rMBP - El Capitan - External monitor blurry
There's a Ruby script here: 

https://gist.github.com/bastibense/6549835

#!/usr/bin/ruby
# Create display override file to force Mac OS X to use RGB mode for Display
# see http://embdev.net/topic/284710
# 
# Update 2013-06-24: added -w0 option to prevent truncated lines

require 'base64'

data=`ioreg -l -w0 -d0 -r -c AppleDisplay`

edid_hex=data.match(/IODisplayEDID.*?&lt;([a-z0-9]+)&gt;/i)[1]
vendorid=data.match(/DisplayVendorID.*?([0-9]+)/i)[1].to_i
productid=data.match(/DisplayProductID.*?([0-9]+)/i)[1].to_i

puts "found display: vendorid #{vendorid}, productid #{productid}, EDID:\n#{edid_hex}"

bytes=edid_hex.scan(/../).map{|x|Integer("0x#{x}")}.flatten

puts "Setting color support to RGB 4:4:4 only"
bytes[24] &amp;= ~(0b11000)

puts "Number of extension blocks: #{bytes[126]}"
puts "removing extension block"
bytes = bytes[0..127]
bytes[126] = 0

bytes[127] = (0x100-(bytes[0..126].reduce(:+) % 256)) % 256
puts 
puts "Recalculated checksum: 0x%x" % bytes[127]
puts "new EDID:\n#{bytes.map{|b|"%02X"%b}.join}"

Dir.mkdir("DisplayVendorID-%x" % vendorid) rescue nil
f = File.open("DisplayVendorID-%x/DisplayProductID-%x" % [vendorid, productid], 'w')
f.write '&lt;?xml version="1.0" encoding="UTF-8"?&gt;
&lt;!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd"&gt;
&lt;plist version="1.0"&gt;'
f.write "
&lt;dict&gt;
  &lt;key&gt;DisplayProductName&lt;/key&gt;
  &lt;string&gt;Display with forced RGB mode (EDID override)&lt;/string&gt;
  &lt;key&gt;IODisplayEDID&lt;/key&gt;
  &lt;data&gt;#{Base64.encode64(bytes.pack('C*'))}&lt;/data&gt;
  &lt;key&gt;DisplayVendorID&lt;/key&gt;
  &lt;integer&gt;#{vendorid}&lt;/integer&gt;
  &lt;key&gt;DisplayProductID&lt;/key&gt;
  &lt;integer&gt;#{productid}&lt;/integer&gt;
&lt;/dict&gt;
&lt;/plist&gt;"
f.close


You'll have to run it in El Capitan's rootless mode for it to work.