Then your installer is not familiar with the Runco Q-750 line at all...
The Q-750i is more than capable enough of lighting up a 84" diagonal screen (horizontally measured as well), unless you are using a screen with a gain below 1.0, which does not make much sense.
In fact, if you look at Runco's own literature on the line, it is recommended for screens between 72" to 92" in width up to a maximum of 108".
I own a Runco Q-750D LED projector and have used it very, very extensively for almost two years with an older high-gain curved silver screen, 4:3 aspect ratio, measuring 76"diagonal (60" width) and the picture is so bright that after calibration i had to keep contrast below zero at -10 (from a range between -50 to +50) otherwise whites would "bloom" severely and become uncomfortably bright.
The picture is amazingly detailed, colors supplied by those Phlatlight R/G/B LED chipsets from Luminus Devices are mesmerizing (no exaggeration), excellent black level with "constant contrast" engaged, no need for yearly re-calibrations, no worries about bulb changes, explosions, costs, availability, and my projector runs very cool to the touch with nearly inaudible fans.
I am sure that if you search around you will find plenty of great deals for the Q-750i projector, which was released some years ago.
According to Home Theater magazine, which tested both the Q-750i and the Q-650i projectors, on a 78" wide Stewart Filmscreen Studiotek 130 screen (16:9, 1.3 gain) the first had a black level of 0.0028 ft-L and onscreen peak white of almost 21 ft-L (very bright) for a contrast ratio of 7.500:1, and the second displayed black level at 0.0049 ft-L and peak white of 17.1 ft-L, for a contrast of 3.500:1.
You should check out the many online reviews for the Runco Q-750i, i think you'll be pleased...
Sony XBR-940E, Integra DHC-80.2, Oppo 103, Sound Lab A-1 electrostatic speakers,SVS PC12-NSD subs, InnerSound Electrostatic Amp,, Hafler 220 amp, Harman Kardon PA5800 amp, Pioneer LD-W1, Darbee Darblet, DVDO iScan Mini, Nvidia Shield, Yamaha CR-7, Koss ESP/950, Stax SR-40, Audeze Isine 10, etc, etc.