in the past (that's no longer necessary as I managed to get lifetime Senator status now), when I was depending on high status mileage earnings to maintain my LH Senator (Star Gold) status, i booked RTW-business and still earned the first-class status miles (at that time 3.75 times, now 'only' 3.25 times the basic miles for first) on UA two-class-flights in my miles&more program.

The best 'mileage-costs' versus 'mileage-earning' ratios on such rtw were my DEN-COS (Colorado Springs) segments, costing something like 37 miles, earning in paid business flying in 'so-called-first' on UA 3'750 status miles per segment.

Nowadays I still pay rtw business (not first) and might upgrade some longhaul segments (like USA-Australia, or Europe-USA Westcoast with vouchers or miles) to first.

The non available first-class in Europe doesn't bother me, as these are mostly flights in the 45' to 90' minutes range. Coast to coast in the USA UA still offers some selected real 3-class flights.