I've had this same question myself. I've asked a couple racers who use them and this is what I've taken from it.

1. It COULD be driven on the street, this does not mean you will WANT to.
2. As long as you are NOT in traffic, almost NEVER come to a compete stop, and NEVER have to start on a hill, you COULD make it work. Still doesn't mean you would WANT to after experiencing it.
3. These setups are designed for racing. You only have to get moving from a complete stop once during most races and that is leaving the pit (no traffic, hills or anyone rushing you)

 

The issue isn't the weight. The issue is the clutch this this flywheel is designed for. Unless you have driven a car with a multi-disk racing clutch you may not understand what I am talking about. The best description is that it operates like a light switch....it is either engaged or it isn't. There is no slip....none.....nada.

I have Part# 02-9207-55NA with a 5.5" Quarter Master dual disk clutch in my race car. The whole thing including the counter weight probably doesn't weigh 12lbs. All of this is wonderful for a race car. But a street car....no way.

I would recommend a light weight steel or aluminum flywheel with a quality performance clutch/pressure plate. I had a set up like this using a Center Force dual friction clutch in my race car for awhile and it would have been very streetable

 

Here's a shot of the Racing Beat 12.5lb Light Aluminum flywheel on my 13b -SE engine before it went in. Note that the 12.5lb flywheel weight includes the counterweight, which means there's considerably less rotating inertia on the outter rim of the flywheel as compared to stock.


Point being, you're getting about an 80% reduction in rotating mass on this one compared to the stock 13b-SE flywheel, which accounts for faster revving and easier gear shifting on upshift as the engine comes down off of revs, and also when downshifting when you blip the throttle to rev match during braking. For street driving, this is ideal where I live - in the flat, open desert - and would probably make it harder to drive if there were any hills around. I've not driven a multi-disk clutch or the 4.5lb flywheel you mentioned, but I couldn't imagine the flywheel being much lighter than Racing Beat's aluminum flywheel. Nearly all of the mass is in the counterweight and very central to the eccentric shaft. That said, the engine now doesn't idle smoothly below about 1200rpm and can bobble and feel a bit lumpy below that, but you do get faster revving as a benefit. I'm glad I went with this part when replacing the engine, as it makes spirited driving a lot more fun.

 

 

I use to daily my 13b semi pp with the Racing Beat setup above with a stage 2 clutch. It wasn't bad at all, I liked it.

Now I have 13b streetport with stock flywheel setup, revs up slow. I can feel a big difference since I was used to my other setup.