Wouldn't it speed it up since the 20W bulbs draw more current (have less resistance)? I couldn't find the lumen output of a 10W 5008 bulb, but these halogen's are about 250 lumens. Brighter is better in this case I believe.Using the 20W halogens front and back will probably slow the flash rate, but not as bad as reducing wattage by using LEDs that do not have the additional resistor included.
Mine were fine as I did LEDs up front plus 20W at rear, therefore total wattage was LEDs (clode to zero) plus the new 20s = 20. I.e. the same as the OEM 10 + 10 = 20, so my flash rate is fine.
This bulb is rated for 2,000 hours vs. 200 for the 5008. I'm not sure whether or not this is an issue though, since it's not on for very long anyway.