[Tunaoue]

Afshinbayegan,
Our cars in stock mode should handle fine if the suspension components are in good shape. Some missing info though; turns at 40mph are no problem if you're on the open highway - obviously it depends on the turning radius. My '84 633CSi turns fine at 80mph on the freeway. So - WHAT do you mean?
Tires start to skid - tells me you're either driving on wet pavement in residential area (hope you're not silly enough to do that), or you need to upgrade your rubber. I recommend 16 to 17 inch rims that are at least 7.5 inch wide. Then get a set of Yokohama AVS ES100 tires -- can be had for about $100 each (which is a bargain for this quality of tire). Many other tires can do the trick too. Dunlop SP Sport 9000 are better handling but wear faster.
Performance -- noticably less power when weather is hot? You probably need the AFM adjusted to correct the air-fuel mixture. Check that you're running with BOSCH WR9LS spark plugs - NOTHING ELSE! Change your air and fuel filters. Check the ignition too, (mechanic's electronic diagnostic test) but Motronic is usually very stable even in elevated temps.
Turbocharging is a popular thing to do. Please realize that you have to swap the motor for another that is setup for the turbo. Least expensive way is to get a 1984 745i and canabalize the engine. These cars can be had for $2500-3000, engine/turbo swap will run another $2000. Remember that the turbo impeller bearings will cause more wear on the oil because of the extreme heat. Have to change about half the normal time intervals. If you're doing it every 4000 miles now, then figure new oil/filter every 2K.

Cheapest bang for the buck: NOx. But the motor will pay a dear price in excellerated wear and tear.

Engine smoke through the oil fill hole? Basically means the motor is worn, lots of blow-by past the rings. Do all BMW motors do this? All BMW motors that are in the same condition will. If your dad has owned many older (10 yr) BMWs, then I understand his comment. It also tells me you should change your oil more often to prevent this kind of wear.

About dad.
I wonder why anyone would call their father "stupid" in a public forum? Look, no one is perfect. We learned as kids in baseball that even the very best players in the league, in the world, will eventually commit errors. We all goof up now and then. It's what YOU or I do in response that strengthens the relationship. I think it wise that you ask questions so that others can help. I wonder since you treat you own father with such care-free disrespect that you also candidly treat others the same? Especially since you dad really isn't that far off from being correct. Your comment of not being able to take turns over 40 - is as bad if not worse.

Suggestion: in the future, just ask your questions. Bashing your dad doesn't help - ANYONE.