Aside from switching to magnetic ballasts (which would be marginal in effect in your case), I personally don't think there's anything you can do to better what you're running in terms of metal halide. That's more-or-less my final answer...
You could save cost by running a different bulb, but you will sacrifice the look. Personally, if you're going to do that - change the look - it makes all kinds of sense to consider setting up an LED replacement.
I am/was in the same boat as you - an extremely happy Radium user. For my buck you can't do better for a halide system. As such, I can share a little it of my own recent thinking.
In my case, budget was the instigator of change. YMMV, but sometimes change is change.
First I considered cheaper replacements and actually tried out XM, Coralife and Hamilton brand bulbs. I have no specific complaints, but none were all that much less expensive than the Radiums (nothing saved me more than ~30%) and none looked as
great as the Radiums did. (XM's were prolly closest.) In my case, I'm running 150w magnetic ballasts (Coralife HQI) so there's also a wattage advantage to exploit (in favor of the corals) in the Radiums too. Anyway...cheaper bulbs could be had on eBay...five for $20, in fact. After my experience, I knew that a cheaper bulb was not what I wanted.
I started to look seriously at LED's... I'd been running about 50w of Ecoxotic Panoramas for a couple/few years on a tank as a personal experiment and had gotten satisfactory results. Personal proof of the technology.
Under initially-more-favorable budget conditions my first instinct was to consider a fabricated, commercial product. Something that had some expertise behind it, saving me some considerable reading and experimentation, not to mention math!
To my eye, Aqua Illuminations SolBlue has a great blend of LEDs in a flexible package that really maximizes their value. (Not being able to vary their height from the water will handicap you a little.) I think this is one's best bet (bang/$ is important) for a commercial LED replacement for a Radium-based halide setup.
DIY is the other main avenue and is more wide open in terms of options. IMO. Options range from the traditional LED-to-Heatsink brick powered by DC current, to the more recent phenomena to adopt the GU10 format bulbs for an extremely modular installation. There are many good kit options among R2R sponsors! Depending on your needs and requirements, in the DIY category the costs can truly range from bargain-basement (<$200 to replace my 300w worth of Radiums) to mind-numbingly awesome rigs such as those created here in the DIY LED forums all the time. Your call.
Prolly more answer than you bargained for....lol.
-Matt
