Don't think you will notice any different. I've got something like that on my car. Maybe made it idle a bit smoother. People say they notice a difference with e-shift on the IS300.
The IS200 standard battery is only 48Ah which is small.
I guess you are talking about the oxygen sensor heaters as a cat doesn't have a heater? The cat heats up from the exhaust gases.
Do you mean higher heat rating? Only required if you increase your engine bhp.
Denso Iridium Power are the best ones to get. Stick with the stock heat rating.
Get them from the US. Lots for sale on ebay.com
You didn't look to hard - they are only two pages of articles in the workshop ;) (I will get this moved to the knowledgebase soon)
http://www.lexusownersclub.co.uk/forum/ind...showtopic=1161&
Do a search on the web for o2 simulator. You should be able to find some.
A better solution would be to get it working correctly still using an O2 sensor. Maybe you need to take it somewhere else?