From the sloper website:
It does provide very good band width for 80m operation. The 80m Reduced Half Sloper loads better then 200khz at 2.1:1 SWR or less when properly trimed/tuned without a tuner.
Given that a typical full-sized dipole at a height of 50 feet will have a 2:1 SWR bandwidth of 250-300 kHz, the above seems to indicate some loss. An antenna as small as the subject sloper without loss would have a much smaller bandwidth.
Consider building your own antenna.