Several things to consider
1. How much is the existing heating and cooling bill?

2. Are you more interested in eliminating the use of all heating and cooling? There are people who want to transform their homes into passive homes and go to great lengths to improve the insulation...

3. How much are you willing to spend?

The only reason I would do if is if it would reduce my heating cooling bill enough to pay for itself in 2-4 years.

Heating costs probably around 300-350/month in the cold months. Cooling 100$ per month.

Since you already have R37 insulation I would think you are not going to save more than on 5% on cooling and heating costs. Most of your loss is through the windows, doors, and walls.

There are very few situations where adding additional insulation would pay for itself in 2-4 years. The insulation companies make these claims but they are rarely true.

It would be unlikely to recover the costs for improving the insulation for the windows, doors, walls, floor. ... In 2-4 years.

Just improving the ceiling insulation alone would have an even smaller effect.

However, the expense probably makes sense / can be justified if you're intent on transforming your home into a passive house.

I would look into what you actually have as far as a unit goes. 8.5 hspf is horribly poor for a 17 seer. Duct sealing and possible upgrading the system might be your best savings.

Did they run a thermal imaging camera and blower door test and do you have any images?

I'm piggy backing on the OP's original post because I am in a similar situation.

I have an empty vented attic (no HVAC equipment or duct work) with blown cellulose on its floor between & covering joists (@ least to code if not exceeding)
This attic sits over our master bedroom, master bath & master closet.

Air in those room always seems to be 2-5 degrees colder or hotter depending on season.

Have had HVAC equipment & airflow checked - all in line along with original blower door test which had us 50% tighter than the comps.

So I think somehow air is still leaking down over this part of my house.
The only reason is that the attic roof area is open to the air infiltration via a passive ventilation combination of eaves, ridges, & side gables & leaking air below to the master space.

Foam would be at least 3x the costs of additional cellulose but beneficially make the attic temp closer to whatever below however, may require additional mechanical ventilation.

Additional topping of blown cellulose would be no problem in an unused space but would it compact in the future & possibly get ineffective in a vented environment & have to be redone?

What is the most effective but cost conscious way to insulate the attic if HVAC equipment is not up there to get my temps back in line with the other spaces in my house?

joshua, your best investment with fastest return
would be air sealing. hopefully the free audit
showed you infiltration sites.

can you give us details of the audit.
what is sq ft of living space of house?
what were blower door numbers xxxcfm @ 50 pascals?
thermal scan with blower door?
duct leakage test?

pbx, starting your own thread would give the OP
his own thread without mixing info between two
posters.
but to answer ...air seal. seal penetrations in
ceiling that attic temps/air are sucked in through
when hvac system is in use.
cellulose would have be be removed for foam
to seal @ attic floor. big cost. big mess.
IF you opt to do this, then air seal from inside
attic, then foam, then insulate on top of foam.

you probably have open thermal bypasss
from attic into the master suite that allow
temp difference,

best of luck,