Originally Posted by WiffleballTony
1) Understeer will be added by the increase of rear grip from the back tires. How bad will it be? Since I am also adding grip up front, wouldn't the overrall grip be better than what I have currently and thus be a net improvement in handling?
It is entirely possible (and quite likely in this instance) to get an increase in lateral grip AND
have the amount of understeer increase. In simple terms, you're still adding more grip at the rear than you are at the front; in slightly more advanced thinking, your rear slip angles will be reduced by more than the front slip angles, and this difference in slip angles will be with you at all cornering g's (not just the max-lateral-g point where one end or the other gives up and starts drifting wide).
2) Am I future proofing myself by going big in the back if I plan on adding really big power adders ie twin turbos etc? Being able to put down the power better out of the turns?
With more tire back there you will have a little extra cushion against a careless right foot . . . unless you also re-balance your suspension for more neutral behavior generally.
"Careless right foot" here means you can expect it to be "trickier" to hold the car on the limit with the throttle without going beyond. You may find it easier to drive if, say, you intentionally use 4th instead of 3rd on corner exit, 5th instead of 4th, etc.
If you really get carried away with making big power it may not matter how much tire you put back there or what the suspension tuning is; you'd just be able to overpower it anyway without even trying.