Sylvania SilverStars and other bulbs with a blue tint are suboptimal:
Daniel Stern Lighting Consultancy and Supply
IIRC your focus uses 9003/H4 bulbs. The best performing standard wattage H4 bulbs you can find are in order:
1) Philips X-treme Power
2) Osram Night Breaker
3) +50 bulbs (Philips Vision Plus, Osram Silver Star (NOT Sylvania Silver Star), Narva RangePower+50, GE Night Hawk)
4) +30 bulbs (Narva RangePower+30, Sylvania XtraVision)
You SHOULD be safe using the lowest power overwatts too (70/65w vs 60/55w - more light than all of above - off road use only
), but there is always the risk of melting wires/housings.
Keep in mind though, HID conversions that use the standard (halogen) housing are a very bad idea:
Daniel Stern Lighting Consultancy and Supply