If your Downstream is in spec & the SNR is good, there would be little on your end you could do to increase the Upstream without causing your Downstream to drop below recommended. Normally it is a "Balancing" issue that your ISP has to look into.
Given your specs, if you increased your Upstream by +10 to 40 dBmV, your Downstream should proportionally drop to -10 dBmV. Not a good trade off. If you have no issues, don't worry about it. If you start seeing issues, you would need your ISP to correct it.
The OP has TV issues not modem. If he dropped his upstream by to from 49 to 39, the downstream goes up by 10. So his average downstream of 4 would up to 14. Less cable and splitter loss increases downstream level an reduces transmit level.