does top end refer to higher rpms or how fast you accelerate at higher speeds?

it refers to power delivery in the upper range of rpms.......like beyond 5 grand...the evo loses some of its oomph up here due to the psi drop-off of the turbo charger.....

acceleration at higher speeds generally refering to 80+ mph

hp at higher RPM increases torque decreases slightly however minor tuning even just a reflash can change that and if you have your car put together right you shouldn't have too much of your boost leaking.