Yes it is a high dose, it would be expected to produce a 'third plateau' trip - 1 being the lowest and 4 being the highest of the four.
Read here: https://www.erowid.org/chemicals/dxm/faq/dxm_experience.shtml#toc.5.6
And our subthread for 3rd plateau trips: http://www.bluelight.org/vb/threads/495571-DXM-Subthread-Third-Plateau-Trips